To beat the first one, you built the trampoline, jumped on the rope, slaughtered the archers, and killed the leader, right? For the second Oliphant/Mumakil, do the exact same thing. Go on the trampoline, jump on the rope, exterminate the archers, and win a boxing match against the leader.
PS: watch out for the archers! They are deadly. While you kill one, the other three almost kill you. I died twice!