Master of Orion II: Battle at Antares/Killing Space Monsters

There are five species of space monster. They are usually found as a "stationary" planetary encounter, but each one of these five species also has a more powerful "travelling" version that can show up at any time as a random event. The travelling versions of each monster are much tougher, usually with up to 4x as many hit points and more weapons as well. These versions usually need well-developed Planetary defenses or strong fleets to be defeated. The game also treats both the Guardian and the Antarans as "monsters". So Scout Labs bestow a 10% combat bonus, while Ion Cannon are equally ineffective against all of them.

Stationary Space Monsters
Usually, the most attractive planets are guarded by one of the five possible species of space monster (for the Orion system, see The Guardian below). Four of the five species have just one weapon that delivers significant damage. This condemns them to being somewhat unfortunate creatures that get wiped out quickly for obstructing a player's expansion plans, rather than posing any serious obstacle.

The key to easily eliminating these foes is attacking with a bunch of small ships. Since most monsters are just able to kill one ship per combat turn, the remaining ones can easily deliver the damage needed to destroy them without the need for expensive protective gear.

The most cost-effective approach is to build frigates with battle pods and armed with 2x MIRVed nuke missile racks. Eight such frigates will kill almost every "non-travelling" minor monster and are cheaper to build than the colony ship needed for subsequent colonization. There's two exceptions, though. The Hydra needs at least 10-12 frigates or one "heavy armor" cruiser to soak up some damage because of the three independently targetable plasma weapons. The Dragon needs 10 frigates since its Pd weapon will take out a lot of incoming missiles.

Tactical Approach to Space Monsters

 * Space Crystal: Only a single ray weapon on both stationary and travelling versions and can telepathically capture your ships as well. Run and fire.
 * Dragon: In addition to the main attack it is also armed with 8 Pd weapons, so best to fire and charge so as to overwhelm it with a missile swarm. The travelling version has 20 Pd weapons.
 * Hydra: it has 3 plasma weapons that can decimate swarms of small ships. The travelling version has 5 plasma weapons. Run and fire.
 * Amoeba: it uses a mid-range Plasma Web-like slime weapon. The travelling version has 2 slime attacks. Run and fire.
 * Eel: it has the equivalent of a lightning shield and a short-range shockwave. The travelling version has 2 shockwaves. Charge and fire at point-blank range.

The Guardian
The Guardian represents some kind of defensive doomsday automated weapon device and is always found lurking in the Orion star system. Destroying the Guardian gives a player access to 4 of eight possible non-researchable advanced technologies, as well as giving the player a specially outfitted Battleship: The Avenger. The Avenger is commanded by Admiral Loknar if one of the 4 fleet-leader slots is still open.

Armament

 * Beam Att 175; Beam Def 100;  Missile Evasion 125
 * Xentronium Armor 800 / Structure 800
 * Moleculartronic Computer (+125%)
 * Class V Shield: -5 + 125 damage blocked
 * Offensive Weapons: 2x Hv Death Rays, 10x Pd Particle Beams, 1x ECCM ENV OVR Plasma Torpedo, 1x ECCM ENV OVR Plasma Torpedo, 2x Spatial Compressors
 * Special Systems: Battle Scanner, Rangemaster Unit, Achilles Targeting Unit, Inertial Stabilizer, Multi-Wave ECM Jammer, Hard Shields, Lightning Field, Automated Repair Unit

Combatting the Guardian
It is quite challenging to engage the Guardian before the mid-to-late game, depending on your technology paths. While it is possible to engage the Guardian in a sluggathon using a large fleet of well-armored battleships with Tractor Beams and beam weapons, by far the quickest method to research and deploy effectively is a much smaller fleet armed with fully modded EMG merculite missiles.

Early descriptions of this approach involved 2 Cruisers accompanied with 3-4 Frigates. But since this layout required Fast Missile Racks, an earlier way proved to be a fleet consisting of as low as 8 Zortrium armored Destroyers, armed with 1 slot of 2x fully modded EMG Merculites. Note the importance of fitting both missiles in 1 slot instead of spreading them over 2 slots in classic, since with the former way, the second missile is guaranteed not to be destroyed by the Guardian's Lightning Field. Part of the combat tactic here is to use Self-destruct to take down the Guardian's shield. It will then likely take 2 successful subsequent EMG merculite hits to blow the engine. Neutron Scanner tech (along with ECCM) will substantially improve your chances to hit the Guardian with missiles. Finally, if the attack fails to destroy the Guardian in 1 round, the shields will recharge 30% and any damage done to the engines will likely be repaired by the Automated Repair Unit.

While spinning the Guardian to death with Gyro Destabilizers was a popular way to take Orion in versions 1.1 and 1.2 of the game, this tactic lost much of its glamour after the weapon was nerfed in version 1.3.

Space marines and boarding parties are ineffective vs. the Guardian.

Prize Technologies
On killing the Guardian, you will be awarded Death Ray, plus a random 3 of the 7 following non-researchable technologies:
 * Damper Field
 * Reflection Field
 * Quantum Detonator
 * Particle Beam
 * Spatial Compressor
 * Xentronium Armor
 * Black Hole Generator

All of these technologies can also be acquired by capturing and scrapping Antaran ships.

Antarans
Antarans will spawn on about Turn 130 and every dozen turns or so thereafter (randomized) unless they have been disabled via setup options. They charge directly at a system, then attack a single random settled planet before leaving. Warp Interdictors will slow down this approach, giving you more time to react.

Antarans use Damper Fields (75% damage reduction; kills 50% of crew trying to beam aboard) with Xentronium armor (10x base armor/structure) but no shields. In battle, Antaran ships are quite tough with very high Beam attack/defense. Thus beam weapons are not very effective until you can get your targeting above 100 or so. Armor piercing doesn't work against Xentronium armor.

Antaran ships are all equipped with a Quantum Detonator, which causes an automatic 3x damage drive explosion when the ship takes lethal damage, and also has a 50% chance of the same explosion when the ship is captured in boarding combat.

Early on, a lone missile base (on each planet in the system) can be sufficient to defend against the first single Antaran frigate, but this will not be enough to ward off later raids. As well, most play strategies forego Missile Base in favor of Automated Factories, which precludes this defense for non-creatives unless you can get lucky trading/stealing for it. If there are multiple planets in a system, the Antarans randomly choose their target from any colonies OR outposts present. Thus it can sometimes be easiest to quickly settle an outpost on a Gas Giant before the Antarans arrive in the system and just hope they attack the Outpost rather than your Colony. Each frigate will generally take out 3 buildings or colonists, so sometimes a colony can survive an attack from a lone frigate with only superficial damage.

By the time of the Antarans' second or third incursion, you should have one or two battleships packed stem to stern with assault shuttles. This will make defeating them very easy.

Antaran Homeworld
Once you have the technology to capture Antaran ships and so thereby gain Damper Field and Xentronium Armor, you are almost good to go for an attack directly on their homeworld. You will also need Dimensional Portal, which can often be gained through tech trade with other races if you don't already own it. Defeating the Antarans here is easy if you have Plasma Cannons and Structural Analyzer.