Here's how mancubuses in Doom 2 work:
Mancubuses don't have flamethrowers, but something that would be better described as "fireball launchers". That's an inherent attack - they didn't drop anything when they died (like barons of hell).
They have fire modes A and B, and they always do A-A-B-A when they shoot, while standing still.
If @ is a player, M is the mancubus, X are the fireballs (which don't have splash damage, by the way) and dots are nothing, fire mode A looks like this:
.....X...........................
..................................
@..X.........................M
..................................
.....X...........................
And B looks like this:
..................................
.....X...........................
@.............................M
.....X...........................
..................................
(note that you won't get hit if you stand still here, and you aren't too close to the mancubus - but the two fireballs are closer to you in B that in A)
Er, this looks better if you imagine the fireballs moving from the mancubus to where they are, in a straight line. :-P
I don't know how this would adapt to DoomRL - probably, mancubus should just shoot at you in mode A. The two outer fireballs could hurt other monsters, or explode barrels (or hit you if you're in melee range, which would REALLY hurt).