Danger

The Mancubus is questionably the most dangerous enemy in game. If you die late in game, it is very likely because of a mancubus (or barrels, or very often, both).

They can kill a full health marine in 0.1s

They fire volley of 3 4d6 fireballs with a splash radius of 4 tiles. If it happens at close range, you can be damaged by splash of two or three, dealing 42 damage on average (84%), with a maximum easily above even marine's base 60hp.

It is interesting to note that the tristar blaster has the same 3 projectile spread rule as the mancubus does. The closer you are to the mancubus as it attacks, the greater the angle of the two side projectile rockets. The further away you are from the attacking mancubus, the closer all three of the rockets are when they hit. I am not certain, but I do believe the angle is calculated to be:

tan-1(mancubussightrange.y/(player.x-mancubus.x) -> degrees

But that is not all.

Blasts destroy walls around you, often exposing you to other enemies shot. Often they finish you off before you can react.

Blast explode barrels around you, which damage you and often place you in puddle of lava.

You are knocked back, which can be both good thing (if you end somewhere safer), or very bad (if you are hurled into lava)

And after a moment - they fire a second volley after about a second at same direction, even if you are already out of his sight. it's friends, exploding barrels around you and hurling you into lava.

And all that can happen in a split second you open a door to mancubi-containing room.

Strategy

First and foremost, FIRE RESISTANCE. Good fire resistant armor can block most of their direct damage and damage from exploding barrels. Fireproof red armor can be made very early. Fireshield and cerberus armor are good options later. Using onyx armor as a base for assembly, or onyx mod on completed assembly is recommended.

If you don't have have fire resistance, each door you open to a room with unidentified enemy can be your last.

But even if you can survive mancubi attack, ideally, Do NOT let mancubi fire at all. They are slow, so it is usually possible to take cover or get away out of fire range. Avoid fighting with them if you have not full health, or near something explosive. Certainly don't reload weapons with long reload time around them. If you are cornered by one, it is often better to hug it and let it attack you in melee (which is not weak either). If it starts firing, or you see you can't avoid it firing at you, SERIOUSLY consider IMMEDIATELY using phasing device, big medpack and/or strongest armor you have in inventory (if you don't wear it already).