Yep as a multiboxer without shamans this is going to be almost impossible without way too much micromanagement. The tactic above works great with shamans. You don't even have to move your guys because when the dragon is in the air his fireball doesn't cause any AoE damage if its absorbed by a grounding totem (confirmed as of a week ago).

I basically just do what the above post said, pull the 2 guards to the top of the ramp and kill them, dropping 5x groundings right as they die, dragon will aggro but rider will not. Dragon will start fireballing me which will get absorbed by totems. I will move to edge of totem range and then when my grounding totem cooldown is up, I will drop new ones and move to the edge of those totem range. Keep doing this till you're right at the edge of the ring, within range of dragon but not rider. Now you just drop your totem load, keep groundings up, spam LB/CL on the dragon. When he hits 40% he will drop, this is when you MUST use your trinkets plus elemental mastery, you can usually kill him before he even reaches you at this point. After dragon dies and you've taken 0 damage, you can now rest or whatever and then kill rider in 3 shots as needed ;D