Disclaimer: I've never fought a gorilla, or obviously a mammoth.
I'd believe it if someone told me a grown human could survive a glancing blow from a gorilla. If he has his focus on you then yeah you're fucked, but if he's surrounded by 30 guys with 70 more waiting to reinforce, his attention is probably going to be a bit scattered.
If you get hit by anything on a mammoth, either kicked or tusked, by sheer difference of mass I expect you're out of that fight. One good trampling tantrum might take out 20 guys who are trying to be in melee range. Mammoth is going to burn through the reinforcements a lot faster I think.
If we're allowed to throw the spears though this might change the entire fight, for both fights.
malicious rattling begins in the distance