This mission can be VERY annoying especially on Legendary! The AI will keep sending Scarab's at you on Legendary.
My strategy for Mission 14: Reactor
The overall focus in order of importance:
1. Protect the Reactor!
2. Get the ODST and max Mac Cannon upgrades ASAP.
3. Take another Covenant Base.
4. Have an army of your preferred units--Warthogs, Tanks, ODST's, and Hornets all do well in massed formations on this map.
5. DO NOT send the Reactor up anywhere, in fact, hide it behind your starting base, until you have taken out all the Covenant bases.
My general walkthrough steps:
1. Get 2 supply pads, then a Reactor, then more supply pads. If you're going for tanks, then get 2 Reactors and upgrade one. Otherwise one Reactor early and Warthogs works well.
2. Get the starting base built up well then take all your units and move up the hill to the first Covenant base. On harder difficulties it's best to get a large army up the first hill--and possibly stick your Spartan's in Tanks.
3. Take out the first Covenant Base--build it up as fast as possible. On legendary a Scarab might show up at anytime, so keep getting more units and towards the ODST and Mac Cannon upgrades.
4. Once you have a max army, move up to the flood base, and annihilate everything. Leave a couple Marines here to make sure the flood base doesn't rebuild.
5. Move up to the 3rd Covenant Base. On Legendary you've probably already had 3 Scarabs come your way.
6. Don't forget to build anti-air turrets everywhere!
7. Kill the 3rd Covenant base with ODST's, Tanks, and possibly Hornets/Warthogs.
8. Lick your wounds, build up that base, and move up the final hill with a massed army.
9. Kill the last Covenant base, take it over, build it up with some anti-air Turrets.
10. Space your units out along the path to the top so they can take out the enemy Vampires aircraft.. that have a very annoying range.
11. Station 2 groups of units at the two Sentinel spawn locations at the very top of the hill next to the Elephant's objective to take them out as they spawn.
12. Bring the Reactor via Elephant up to the top along the now safe pathway!
The Vampires spawn on the top right of the map, near the 3rd Covenant base. It's hard to intercept them however, so spreading your units around is helpful.
The Scarab's are TOUGH on Legendary. Hit them with everything--drop ODST's around them so they can't move then use the Mac Blast, Canister shells from the tanks. You'll probably lose half your army every time a Scarab shows up.