You can quite easily kill a deathwork by spit roasting it with two rock elementals. Just have one on either side of the work to keep it still and smash away. The elementals will probably take up to 2k damage each.
Tame a 100+ megatherium and enrage is by attacking creatures that give you chitin. This will give you a damage boost and a damage resistance boost. Level up melee and you will easily be able to kill a death worm