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 hitpoints 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 it 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 missiles 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 it's 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. 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 and can telepathically capture your ships as well. 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.

Antarans
Antarans will spawn on about Turn 100 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 (1/4 damage and Neutron Beams/Deathrays do not kill their crew) 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. At that point enveloping (base 4x damage) or armor piercing weapons can work (minus reduction for Damper Field). However, Antaran ships are hard-coded to self destruct with enough damage or if they are rendered immobile. So EMG-MIRVed Nuke missiles have a very good chance at destroying their engines, thereby causing a self-destruct. If enough Tractor Beams are used to render them immobile, this will also initiate a self-destruct. Achilles Targeting Units and/or Structural Analyzers can also work well in the late game against Antarans, since they have no shields. Neutron Scanners or better will be necessary to use any missiles against the Antaran battleship-class or higher. Augmented Engines give you that little extra distance to get you in range to take them out quickly.

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 between any colonies OR outposts present which one to attack. 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 early-mid-game using ships against them is possible with the right tech: EMG-MIRV nukes or Tractor Beams with enough defensive systems/armor packed in to survive for several rounds of combat should be fine (Reinforced Hull + Heavy Armor + Zortrium armor or better). Just move up next to the Antarans and fire your EMG missiles point-blank directly into their ships to blow the engine. Tractor beams allow you a bit more tactical freedom, as you can immobilize the Antaran ships from a distance to trigger their auto-explode.

Once you've perfected the art of immobilizing the Antarans, if your troop combat rating is high enough, you can try and capture them as well before they go boom. Their Quantum Detonators means there is always a 50% chance of setting off the auto-self-destruct by mistake even on a successful capture. Generally with Personal Shield + Zortrium armor + Phasor Rifle you have an even chance of capturing an Antaran ship if you swarm it and board with 5 or 6 of your own of the same or larger class. Alternatively, if you are using Assault Shuttles, you might be able to muster enough boarding marines to overwhelm the Antarean crew in one large attack.

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. Getting into a fire fight with the Antarans is a recipe for disaster. It is much easier to simply take out their ships through the above-discussed auto-destruct method, and capture the Antaran star-fortress by zerging it with commando boarding parties. You should also have at least Powered Armor, Personal Shield and Plasma Rifle for your troops, as well as a Leader with a Commando bonus. With the above listed tech, the Antaran Homeworld defense force can easily be defeated with the following modest strike force:
 * 6 Battleships: Battle Pods, Troop Pods, Reinforced Hull, Damper Field, Automated Repair Unit, Augmented Engines, the rest of the space filled with MIRV ECCM EMG merculite missiles.
 * 6 Battleships: Battle Pods, Reinforced Hull, Damper Field, Automated Repair Unit, Augmented Engines, the rest of the space filled with 10 miniaturized Assault Shuttles.

Note that the Automated Repair Unit is just there so that taking any damage less than concentrated fire from the Star Fortress will be shruggable. You still have good chances for success without it.

Fire your EMG missiles point-blank into the sides or rear of Antaran ships to quickly destroy them and clear the board during round one of combat. Advance all ships towards the Star Fortress. On round two swarm the Star Fortress with ships, preferably those having Troop Pods. Any ships close enough should engage in boarding Raid parties to try and take out key defensive systems. Once the Security Station is destroyed, simultaneously launch all Assault Shuttles. On the next round your massive Assault Shuttle attack strikes home and overpowers the Antaran defenders. You win.