A large pig-spear trap or crossbow trap would be my solution to this problem. Has to be simple, but able to pierce the heart of a mammoth.
"Simple" rules out pitfalls because they'd take ages to dig and prepare for a single mammoth. You could look for natural places to make a pitfall, but that's not likely, either.
Honestly, a simple pike made from a thick tree and anchored in the ground would probably do all right. Piss the mammoth off and get it to chase you right into the spear. Just have a fallback plan in case it doesn't work. Like running for your life.