As red on Craters, you’ll often be allied to green. You should get the firestorm stonehead (though green may get it, it’s usually better for red to get it), and will have to keep in mind that blue likely doesn’t have firestorms. You’ll want to focus on attacking blue, and make green double him. You can back door blue at any time.
As quickly as the game starts, charge only convert and keep converting until you have about 25 people. Then start charging landbridge immediately, and use it as shown above. You’re competing for control of the erode stonehead area with blue. The erode itself is not that interesting, but the control of that map area is. It’s good to make green do 1 or 2 LBs out of your backdoor to blue as quickly as he can. One common strategy is to instruct green to train troops (pooled in your base until they can go to blue) and then train troops yourself. Then backdoor blue as soon as possible. Use blasts and ghosts or a lightning to kill his shaman. Warriors will be useful to attack his huts, and firewarriors to camp his reincarnation site. By the time you get your first firestorm, you could be landing it on blue already. If you get blue’s firewarrior hut and camp his reincarnation site with firewarriors, killing him should be easy. You can kill blue in less than 10 minutes from the game’s start. In that time, you could even have volcano from the map center’s stonehead!
Your ally green can land bridge to the erode stonehead area and attack blue from there while you attack from the back door. Both of you should be using warriors.
Blue’s hills can easily be land bridged, and, even more destructively, you can get on one of his hills and then landbridge another hill.
Mid-game, consider filling in your crater with landbridges in order to build more.
Backdooring blue early with FWs, warriors and volcano (blue is playing on red’s land, red is playing on blue’s land)