On the level " the Outskirts" you will make your way through the level until you pass a area known as "Zanzibar Hotel" once passing this if you continue forward into the drop area where the phantom drops a load of covenant look down to your right. You will see a small little area under you that has a few potted plants in it. This is where the Invincible Warthog will spawn.

Continue through the level until reaching the last possible checkpoint in the level. (Typically I just like going to the final tunnel), once doing this go back to the previously mentioned area, and you should find a warthog you might need some grenades to get it, but most of the time you can just drive it out over a little bench looking thing.