Depending on what you're doing, your best bet is to go with Hawks. You can get ODST's but they don't move around quickly enough IMO. If you're going for the skull or achievement, do those on two separate play-throughs so you can specifically focus on them.
As for the Hawks strat, upgrade your base, make about 4 sp's, and let your Spartans take care of any enemies that may advance on your base. After you've killed off the initial flood attack, send a ODST squad to the forerunner sp to the left of your base and another to the second forerunner sp near the second control terminal located at the south-east portion of the map. When you've made your 4 sp's, upgrade your base, and get some reactors up, followed by a field armory for reserves and reinforcements (since you're on normal you don't really need to worry about the turret upgrades). Also, don't forget to upgrade your existing sp's to hsp's.
When all that's done, build a airpad, make 3 hornets and put your spartans in them. These 3 will easily be able to defend your base on normal. After that, start upgrading your hornets, but you will need to have upgraded both reactors beforehand. When you've actually 'purchased' the hawks upgrade, you can either sell your second reactor for a supply pad or another airpad to double pump hawks; considering hawks are only tier 2 to make and you only need 1 upgraded reactor.
Finally, once you've hit the max pop count you can quite easily take out any scarabs or bases left so that you can activate the control terminals.
Using this method, I can finish anywhere between 15 to 5 mins before the time limit ends, depending on how quickly I act. Considering I always use this method, and I've pulled it off each time, you shouldn't have any trouble with it.