Trash mobs were the hardest, much harder than the bosses. I like to stay hugging the left wall by the entrance and pull the casters in via Line of Sight. I use WindShear to pull them close. I also re-glyphed my tank to use Glyph of Turn Evil which allows me to kite an undead caster via instant-cast fear.
The escape from the Lich King was hard as well. I'm relying more on magma totems for the trash mobs and I summon one elemental at every pull just to help with aggro mgmt.