Towards the end of the game, when you have collected the Tyranid samples, located the Hive Fleet and acquired the forge you will be given several days to await reinforcements before you can attack the Tyranids. During this time many missions appear, including Raid Against the Warboss.
The mission is pretty straight forward. Get to the opposite end of the map and kill the warboss. Keep in mind that the warboss is very deadly, and a lot harder to dispatch then other bossess.
Be sure to capture the strategic point just below the location of the boss, as you will need to use this to reinforce your squads should you need to fall back.
Apart from this, just throw what you can at him, fall back, reinforce and repeat. Just keep in mind that he will slowly regenerate health when not in combat.
The warboss has several special moves.
- He has a ripple special in which a marker pointing away will signal, followed by an attack rushing out along the ground
- He has a shockwave attack signalled by a circle around him
- He has a bombardment attack signaled by a large circle with red shapes inside it
He can also throw multiple grenades.
The warboss is especially vunerable to plasma fire, so it might be wise to have your devastator squad stand afar with a plasma cannon, and use another squad to keep him back.
As a final note, the warboss periodically calls in reinforcements from the cave behind him, ranging from basic units to heavy squads to vehicles. I used a scout squad, and had remote detonation packs at the cave to take out as many reinforcements as i could, as well as the added benefit of power shots given you have the ability and the sniper rifle equipped.