Pistol
This should be your primary weapon if you haven't collected any other. Simply shoot at the helicopter and dodge shots. Its accuracy is relatively poor, but with the unlimited ammunition you can often take out ground enemies with this anyway.
Knife
Practically useless for helicopters. What it can do, a grenade can do better. However, you can quickly destroy ground enemies with this. Note that once an enemy is destroyed, the score you receive is equal to the amount of damage the enemy sustained times 100, so the knife will grant more points than the bow and arrow or pistol.
Bow&Arrow
Inflicts more DPS than the pistol at close range when every shot hits; however, it's often not worth the time to switch through your weapons unless TriDamage or TimeSink is on.
Grenade
If no better weapon is available, spam these at helicopters flying very close to the ground. Troops can be taken out easily. Keep in mind that you do have a limited supply.
MachineGun
Destroy any enemy (ground troops included) mid-hyperjump to gain 25 ammo to this weapon. This can be done with any weapon, and will automatically give you the MachineGun if you haven't already gotten it from a pickup. Attack ground enemies with this weapon more than usual, since ammo can be easily farmed.
AkimboMac10's
Use when you're in a particularly tight spot with an enemy and need it gone fast.
ChainGun
Don't try using this on a ground enemy unless it's in the way of your target; you should use the slower-firing weapons for this, as it is easy to oversample your shots. Instead, use this preferably only on Big Guns, as it allows most, if not all shots to hit.
Shotgun
Take out mainly ground enemies with this one if possible. It takes a while to destroy a heli with a Shotgun, so do not use if your time powers are low.
DoubleShotgun
Because of the large spread, you should use this only on Big Guns. More so than the regular Shotgun, you can shoot this weapon at an angle such that some bullets hit a ground enemy. This weapon takes a while to reload and to destroy enemies, so use even more caution when using this weapon for extended periods.
SniperRifle
It is actually more difficult than you might think to use this against a ground enemy. With bullets flying everywhere, this is not a game in which you want to spend aiming at things too long. Use one of the adjacent weapons to take out ground enemies, and watch your time powers.
GrenadeLauncher
This weapon has been maligned a lot in HA2, but in fact each ammo pickup can dispatch 3 helis, which is above average. You also can remove 2.5 Big Guns in HA3 with this package. The Grenade Launcher is incredibly versatile, as it works to destroy anything quickly, and another bonus is that even if it misses its target, it may still bounce around back to the target when it explodes.
RPG
Don't try this directly on ground troops unless you have lots of open space and can aim downwards. On the other hand, this works great when you can do this, and whenever you can shoot a heli low enough so that the blast will destroy a ground enemy as well, as it runs direct and reliably to its destination. The RPG usually takes 5 grenades to kill, although it may take up to 6.
RocketLauncher
This is usable against ground enemies, but the grenade launchers are better suited for the job. Instead, knock out the helis and you might transfer the blast to ground troops. Because of its medium-high damage, fairly large ammunition supply, and good fire rate, the Rocket Launcher is best suited to taking out larger helicopters where the rockets are less likely to miss.
SeekerLauncher
This weapon has less ammunition than the standard launcher. However, it's more suited to destroying ground enemies and is much more convenient to use, as you only have to focus on dodging the shots. If you can switch to the DrunkenLauncher directly afterwards, you can give a devastating double knockout punch, as two Drunken and one Seeker will destroy a Big Gun.
DrunkenLauncher
Don't underestimate this weapon. Two perfect hits will leave even a Big Gun within death's fingertips. Either use at close range, or fire at a distance and hope for the remaining rockets to hit a stray target, or the next heli.
GuidedLauncher
The GuidedLauncher would work great against low troops except that it requires extra concentration to reach its target. Against troops who fire extra shots, this is simply not a luxury that can be afforded most of the time. Usually you will want to use this against ground troops that are far away, destroy them with the blast from a hit to a heli, or direct the missile into a wall near the ground target and let the splash damage take care of it.
ShotgunRockets
This weapon retains good attributes of both of its counterparts with few downsides. Consistent spread makes it arguably more useful than the DrunkenLauncher. Use it on the helis and try to get ground troops with the blast. Four rocket salvos will destroy a Big Gun.
FlakCannon
A very powerful shotgun-like weapon to use against helis very close ranges. A couple of shots can take them out, but at anywhere other than close range, it is difficult score hits, due to the random velocities of the projectiles. Try to use this in a hyperjump while aiming downwards, for the chance of a stray projectile hitting a ground troop.
GooGun
An excellent weapon for killing ground troops, as the goo behaves like mines. It also works great on helis. A near miss may also end up destroying a ground troop anyway. This is a better weapon than you might think.
SparkPlug
Another weapon well suited to ground scouting due to its high volume/damage ratio. However, the shot can be spread between a heli and a ground troop on occasion.
FlameThrower
Leading your target is important, as the flames travel slower than most projectiles. Using the FlameThrower at short range guarantees the longest duration of contact with an enemy. With effective use, one ammo pickup can destroy multiple helis.
Bladerang
Try to shoot at a 30 degree angle. Typically, you should be able to destroy a Big Gun in two shots due to the projectile dealing damage as long as it is in contact with the heli. If it takes any more, use something else; occasionally, if you pull it off correctly, you will knock them out in one.
LaserRifle
This is fairly accurate with fast damage, and can be used in any situation. It's damage potential is situational, but is often seen as an upgrade to the Sniper Rifle. The number of additional hits scored depends on the time spent by the laser in contact with the target. Remember use angled shots and time powers when possible for maximum damage, as with all laser weapons.
AutoLaserRifle
This is less accurate, so should be used more in contests of raw speed where you have to knock out the heli fast compared to the more consistent Laser Rifle. Due to its lack of accuracy, shooting from the side is less effective.
ShotgunLasers
This can kill in three shots if shot from the side, and can be done frequently due to the fast reload of the hyperjump. You can easily shoot through a ground troop as well as a heli. Like all other laser weapons, damage of additional hits are affected by TriDamage.
RailGun
A powerful weapon that can shoot through solid objects. Extended use should not be attempted without some space to dodge. This is especially true against a Big Gun, which requires three shots to take down. Using time powers will greatly aid in aiming, due to the hitscan (instant travel speed) nature of the weapon. It is fairly difficult to set up a shot through a ground troop on the way to the heli for extra points, and is best left to other weapons.
Anytime
This Anytime is difficult to master. It lacks consistency, and may take more shots to destroy a heli than the Rail Gun. For more damage, the Anytime should be shot to the side of a heli at an angle similar to the Bladerang/laser weapons. This causes the projectile to be in contact with the heli for the longest possible time. This is because the Anytime's projectiles create explosions at regular intervals while in contact with an enemy or destructible environment. To make the Anytime more effective, use time powers, specifically TimeDistort and TimeRift. By using TimeDistort/TimeRift, the distance between the explosions is reduced, allowing more to be stacked on a heli. Combine this with an angled shot for consistent one-shot kills on almost anything, including Big Guns.
AirStrike
Difficult to use against helis, since the direction that the bombs will be dropped (left or right) is random. As a result, it is a very inconsistent weapon. However, it will purge ground enemies very quickly, and can get a lot of MachineGun ammo via hyperjump.
Taser
Don't just shoot it and wait until the heli goes down...remember that you can switch to other weapons to deal more damage for a quicker kill. This can be done as soon as you shoot the Taser or while the heli is getting shocked. Time powers works better with this than most weapons, as you can focus more on dodging bullets.
Soundwave
This is probably the best and most useful weapon in the entire Heli Attack saga. Flamethrower may be able to do more damage eventually, but anybody can master Soundwaave fast. Shoot from the side and since it can go through walls, you can take out enemies as well. Typically, 5 or 6 helis can be destroyed with one pack of Soundwaave. If you have TriDamage...I won't even go there. The point: Helicopter pilots don't like bass solos. But you should.
A-BombLauncher
These are slow, so either jump up and blow the heli into the Stone Age as in HA2, or fire the bomb from a fairly low level and hope for the fallout to annihilate a ground enemy as well as the heli for additional score. You can, in addition, shoot at a ground enemy instead and heavily damage a heli to get more points out of one shot, but opportunities for this are rare. With some luck, it is also possible to knock out multiple helis at once with the fallout. Using this weapon under the effect of TriDamage is not recommended (as other weapons could benefit from it), unless going for points.
BlackHoleGenerator
Just jump and suck them into the abyss. Wait until there is a full complement of ground enemies before you fire this, because then you can get large amounts of MachineGun ammo via hyperjump. Note that any enemy bullets, as well as anything you shoot while the black hole is active can and will be sucked up, including RailGun shots and A-Bomb Launcher rockets! The most important rule: do not hold onto the BHG for very long. If you get additional ammo, you have to reload it without using any other weapons! If you do happen to get stuck in such a situation, use time powers to help with dodging.
FireMines
A returning weapon from Heli Attack 2, FireMines have improved in both damage and utility; a single FireMine can destroy up to two of any heli with proper timing. Upon being thrown and touching the ground, a targeting laser will point upwards. Any enemy that crosses the laser will cause the mine to detonate, sending a column of flame upwards (limited height) for 3 seconds which can also pierce walls. Mines will also detonate automatically 5 seconds after being deployed, or by moving off-screen (like most explosives).
AntiMatterCannon
Fires extremely high-damaging waves as fast as the ChainGun. Each shot deals damage equal to half the health of a Big Gun, allowing you to destroy up to four of them with a single pickup. You should be concerned with scoring hits and not oversampling your shots, as the projectiles travel in a wide spread at a speed slower than most projectiles. Use TimeDistort to slow down the fire rate and avoid wasting ammo, as you only get 8 shots per pickup.