At least 1 engineer, and then any other character/trooper (best if the trooper has anti-tank weapons).

Go in the tunnel, all together. The engineer repairs the 2 others while they clear out the tunnel. Clear any C4 you encounter. Then, when you reach the far end of the tunnel, which is near the power plan, stay there.
Now, here's the flawless tactic. Have the less expensive trooper run straight ahead, and the engineer and other dude (if he's still alive) follow. The guy who went in first will get shot by the obelisk, but the others will be able to move unharmed.
What happens next? You enter the power plant, boom.

[ March 03, 2002: Message edited by: PyroTeknik ]