So earlier today I used Hydro Pump with my Greninja. Then I died from Toxic and sent out Dragonite, used Dragon Dance and then Fire Punch, and Aggron died.
After that I wondered, does Metal Burst take Multiscale into account?
This is because Metal Burst does a set amount of damage. It will always return 1.5x the damage the user took, similar to Counter and Mirror Coat. The target's type or ability has no effect on the damage done.
So if Aggron took exactly 100HP in damage, then your Dragonite would receive exactly 150HP in damage from Metal Burst.