I just pulled mine off 2 times in 2 games making each game 1-0 us. it's REAL simple and if you do it right it should work at least once on each set of opponents.
To put it simply
1. 3 people in warthog immediately, other goes sniping if you have another.
2. Drive around the ribs and have the PASSENGER get out right before the big rock mass to YOUR left of their base.
3. Continue driving and get to the back of ther base under and around the bridge thing. Have the gunner shoot all over the place while you drive around in no inportant pattern
4. While step 3 is being done, you passenger should have hopped up from the front of the base, grabbed the flag and jumped out the SIDE window.
5. Flag guy runs off the cliff, under the rocket launcher(that direction)
6. Warthog guys will die and respawn at the base where they can head towards the flag guy and defend him or inch the flag along if need be.
Try it out. Guarantee you it will work if done right. Ive been through TONS of Burial Mounds games and I know they always end in a tie and I have tried everything but this is the ONLY thing that has worked.
Oh yeah I just realized I was talking about CTF. Assault is the exact same process except you want to have the passenger get out and plant the bomb, not take the flag.