The best offense is a good defense, right?

The tougher areas are going to be impossible to explore solo no matter what forms you get...may as well build something that can tank just about anything anywhere with the right group.

You'll want porcupine. You can get a lot of mileage out of porcupine and pachyderm/tortoise/armadillo. Throw in a regen form along the way, something like hare that can rescue, and you're set.