Diablo198 thats why you take backup with you i.e. the shotgun soldier. If you make it to the obelisk set the C4 and stay their. C4 takes 30 secs just before this you detonate the remote explosives.The shotgun soldier kills anyone coming in. Noone will have any special characters and it will waste time even if they get you.
I have done this many times.
Rorschach