with your lmg
i think its natural instinct to shoot and try to kill any enemy in sight. most of the time this is the most effective way to kill anything but its not the case with the onos. too often i see marines spamming their lmg's dry at an onos in an attempt to kill him but he just retreats to heal up. some marines even try to chase him but because theyve probably exhausted their ammo theyre probably only left with a knife or pistol which isnt enough firepower to finish the onos off (and if he has regeneration hes probably gained another 50-100health). Because of the huge res cost for an onos theyre generally quite cautious and will retreat quite early. The key to killing them (this is with lmg's btw) is to keep the onos exposed as long as possible while hes trying to escape. most available marines should take high ground out of range of meelee attack or try to stay as far away as possible or even behind boxes - anything that will slow the onos down is good. then try to draw the onos out possibly by baiting with a marine or just letting him come out unknowing how many marines there really are. then the important bit. Dont shoot at him yet. As long as the onos isnt being shot at, he wont retreat. when hes waddled in far enough (when it looks like hes gonna retreat or is getting to close ) everyone should open fire. after he takes a bit of damage he'll probably turn back and try to get away only to find himself dead before he can reach cover :>
another thing to try is to block his escape with a brave marine. the marine will probably die after 2 seconds but thats another 10 shots from a couple of lmg and could be enough to finish the onos off.