MT should Tank Behemoth DEAD North at the edge of the arena. (His knockback move actually puts you there anyway), dodge the narrow red ground-effect cone, and rotate your cooldowns.
Job Done.

OT should Tank the Giant to the EAST or WEST of the Arena (I usually go East... since the giant will pop North, then after the first roar East, then after the second roar South)
I've never seen a Comet drop on the MT, but I have on the OT.

In dire circumstances both Behemoth and the Giant can be tanked by the same tank (I've had to do it before, it's doable but you really need decent healers backing you up)