I completed mine about a month ago. The hardest part was farming the ice burrower meats in Western Wastes. The rest was relatively a breeze but consider I was level 75+ when doing it all. Final water elemental easily soloed. Had a full group of 80 guildees do earth and fire, then picked up another full group to do air. Total time for the last three was about 3 hours.
On a related note and because I don't want to make a new thread... How good is the water elemental form's mana regen? Is it similar to being on a horse out of combat?
No no no. I've bought the first level just to have something in non-mount zones, but it's only 1 mana regen per level (max 3) so it's nowhere close to being on a mount. The way I see it, though, something is better than nothing.