Hi,
Do you mean the draaq boss?
If so, get out your rocket launcher, and continusly fire at it with primary fire.
When it splits, ignore the thing that fires at you(try to dodge its shots though), and concentrate on the other part with the "ball" under it. Use the secondary, tracking mode on the second part(w/ the ball), cos it`ll dodge your primary rockets. It`ll then join together again. Just repeat the above again until its dead.