How do I run an adventure with the party in competition with an NPC party in a race for the MacGuffin?


Hoping for advice:

I’m looking at the next leg of our campaign being a “race for the MacGuffin” scenario. The PCs (Rolemaster) have been chasing an evil sorceress for a while now. I want to let them catch up but put them in a position where they can’t actually kill her. She’s working for someone powerful who will come after them if they kill her. But it’s in a mercenary role so if they can get the MacGuffin for him he won’t care what happens to her after that.

The problem: I’ve never run or even read an adventure like that. Can someone point me to some good resources? Advice from the geniuses here is considered good as well.