Their Shadow Priest dispel-protects through your CC - focus the healer directly rather than trying to burst the DPS first.
Fear can break your chain mid-burst - stagger CC (Warrior stun, then Mage root) instead of using both at once so one always lands.
Whoever lands the first full Warrior-stun-plus-Mage-root combo on the enemy healer usually opens the deciding burst window.
This is a favorable grind - Mage kites the Paladin while Warrior and Druid pressure the Shaman healer.