From Halopedia, the Halo wiki
Enter a 1v1 death match, you as Cutter and opponent as Forge. Send your first warthog to the back of the opponents base and repeatedly spawn them and attack with the Warthogs (only spawn 9). He will try to build a barracks and a vehicle depot, but just destroy them as soon as they come up. You should also get the Big Al's Scooter achievement also as it is obvious that you get this in under 10 minutes.
Enter a 1v1 deathmatch as the Prophet and the opponent as any leader on Blood Gulch. At the start of the match, quickly create a Temple and send your Type-32 Ghost towards the enemy base as a distraction. Once the Temple is created and Regret is on the field, send him towards the enemy base and use his cleansing beam to destroy the enemy base and/or any enemy units that pose a threat to him. If done efficiently, you should be rewarded with the 'Big Al's Scooter' achievement as well.