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 timebreak 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
Make sure to get the extra 25 bullets associated with a hyper jump when possible. You can get these by killing anything, rather than just a heli. Using the BlackHoleGenerator will get you a lot of these spare bullets. Attack ground enemies with this weapon more than usual.
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 anyways; you should use the slower firearms for this, as too many bullets will miss and be wasted. Instead, use this preferably only on Big Guns, and it is a great weapon to use if you need new weapons or health fast.
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 cartridge 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 make 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.
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 fingertip. Since no large-volume weapons are directly nearby in the running, either use three shots and hope for the remaining rockets to hit the next heli, or use something else to start and finish off the heli with two shots.
GuidedLauncher
The Guided Launcher 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, or destroy them with the blast from a hit to a heli.
ShotgunRockets
This has the good attributes of both of its components with little of the bad. Use it on the helis and try to get ground troops with the blast.
FlakCannon
This weapon should not be used near ground troops, as it is very powerful against helis but not against the ground enemies. It can take them out, but at anywhere other than close range, it is difficult to shoot. Try to use this in a hyperjump while aiming downwards, such that a ground troop may take a hit that doesn't find its way to the heli.
GooGun
Kills ground troops with abandon, and it works great on the helis as well. A near miss may also end up destroying a ground troop anyway. This is a better weapon than you might think.
SparkPlug
Another weapon better 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
Be careful with where you aim this weapon, as the ability to destroy multiple helis with one pack is the sign of any good HA2 or HA3 player. Ground enemies have no chance, and it's not worth the risk of trying to aim at close range. Shoot from a high perch, a high jump, or both, and try to shoot across instead of up.
Bladerang
Try to shoot at a 30 degree angle south of west or east. Typically, you should be able to destroy a Big Gun in two shots, and 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 agreeably accurate with fast damage, and can be used anywhere. It's generally superior to the sniper rifle, as it boasts the same degree of accuracy with less shots to kill, more than twice the ammunition, and a slightly faster fire rate. Remember to shoot across on all lasers if possible.
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 tactial Laser Rifle. Due to its inaccuracy, 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 hyper jump. You can easily shoot through a ground troop as well as a heli. This weapon is dynamite during TriDamage.
RailGun
This is fast to shoot, but extended use should not be attempted without lots of wiggle room, as it takes three shots and it is difficult to set up a shot through a ground troop on the way to the heli. You should resort to the lasers if trouble brews.
Anytime
This weapon is the sign of a true expert. It should be shot from the side at an angle similar to the Bladerang, but slightly closer to the equator. This is best done right as the heli is leaving the screen, when only half of it is visible. This will land the most consistent one-shot kills; on rare occasions, a plain side shot will also achieve this. Otherwise, Anytime will destroy a big gun in 2 hits. Unlike in HA2, reload times reset when you move weapons, so don't use the Anytime if you do not have a plan to outrun helis. If you have Invulnerability, empty this out, please.
AirStrike
Doesn't do much against helis, but it will purge ground enemies very fast, and it can get a lot of machine gun ammo if you jump at the right time, especially if you can jump directly towards the heli and finish it off with a heavy-duty weapon. If you do plan on using this principally for destroying helicopters, call in 2 and if that doesn't destroy it, call in another one.
Taser
Not much to this one, other than grab and hold tight. TimeDistort works better with this than most weapons (TimeBreak is typically the best power to use otherwise), as while you are dodging bullets, the heli still gets shocked.
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. 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. Don't shoot at a heli with this during tridamage; use instead the aforementioned tactic.
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 the maximum MachineGun. Because of the ammo you can get out of this, the net destruction per black hole can extend well past what you suck in initially. Try to use this weapon above any other when you get surrounded by bullets, as all that will be sucked up as well. The most important rule: do not hold onto the BHG for very long. If you get another, you have to reload it without using any other weapons! Try dodging shots for 19 seconds with limited TimeBreak. It's very tough.