I'm not the best halo player, and I don't get a bunch of headshots etc which a lot of these solutions seem to require. I like grenades, grenades are awesome.
Here's the modifiers I chose.
Pinata (melee a grunt, get a grenade!)
I die quite a bit, it's much better to quit and resume when you die (before you respawn). May as well take the free bonus from the iron skull.
I kept the sniper with me throughout, it come sin handy on the hunters, who are a pain, and taking out some of the cannons early on. I kept a plasma pistol as my primary throughout too. Nades were key for me, get a good grenade and you'll clear a bunch of enemies and rack up a big score boost too! Just watch out for any explosions near you, with boom on you'll get caught off guard quite a bit, but it's worth it. Other than that just take it slow, do the quit resume method as often as you need and you should make decent time. When you get a chance, punch a lonely grunt to get some more plasmas.
I did it in 28:12 which netted me just over 2x bonus from the time giving me a score of 35557. You notice with that score I had plenty of time to go even slower, you only need 19,000 now after they lowered the scores.
Hope this helps, if I can manage it (it took me an hour maybe with all the quit/resumes) anyone can!