Mission 8 is mostly VERY easy, and can be finished quickly. It's basically an introduction to the Flood.
My strategy for Mission 8: Ander's Signal
The overall focus in order of importance:
1. Hold off the Brutes attack
2. Rescue the first missing Elephant
3. Build and protect your base
4. Rescue the last missing Elephant
5. Protect all the Elephants from being destroyed!
My general walkthrough steps:
1. Lock down the Elephant immediately, and start spamming half Flame-Throwers and half Marines.
2. Send Forge forwards to the little rock formation, and send all your Marines and Flame-Throwers there as well.
3. The Brutes will start dropping in--use Forge's Shotgun ability, and take them out as fast as possible--sometimes they target the Elephant and it's very weak--also, if you lose it, you lose!
4. Hold off the initial waves of Brutes, then start advancing using troop abilities as much as possible.
5. 3 Spartans will come down your way to help out, and you should be able to clear out the Brutes fairly easily now.
6. Move north(send a couple troops to collect the resources on both sides there), and bring the Elephant up to the plateau.
7. Lock the Elephant down near the left exit to the plateau, leave any Marines you have left with it to guard, then send Forge and the Spartans towards the top left Elephant.
8. Gradually approach, let your 4 heroes take out any flood slowly, but do focus on taking out the Flood Gob Thrower, and eventually liberate the Elephant.
9. Escort the Elephant back to base with a slight detour to the North-East from where it was to rescue some ODST!
10. Head back to base.. or send the Elephant to lock down next to the other Elephant.
11. Start building up your base quickly, with Turrets that have anti-infantry upgrades.
12. Get a Field Armory up soon and research upgraded Turrets.
13. Rescue some more ODSTs--they are scattered around the left side of the map, pretty much wherever you see the Flood "Pods". I believe there are 4 groups of them to find.
14. Now, you can either focus on protecting your base very well--Hornets are the best--or make a run for the final Elephant and win quickly.
15. If you go for the final Elephant, try to keep your ODST's between or behind your Spartan's and Forge.
16. Rescue the final Elephant, and walk back to base!
17. You must keep all the Elephant's alive, so use the Heal Power on the ones near the base if necessary, and possibly train more Marines out of them to protect them.