From Halopedia, the Halo wiki
Titan is an Achievement in Halo Wars that is unlocked when the player receives 20,000 points in any Skirmish game.
It is worth 15 Gamerscore.
Play on Release on either Deathmatch or Standard as Anders on Easy. Make Hawks to fill your population capacity.
Send half to Southern Release area and half to Northern Release area. Put Spartans on the Release Controls.
Release the Flood periodically. There is a cooldown period between each release. Keep doing so until you get 4000 points and then attack enemy base.
Don't activate any skulls and remember that the enemy will sometimes attack your base.
Max points actually became 4000 per game due to a Title Upate.