Blaze is a "AOE" unit, his main role is to damage the croud applied burn effects.
Passive: Burning Hit
apply a burn effect that does damage over time.
Active: Make It Hell
Reduce all healing effects for a short period
Tactics:
passive - since "Burning Hit" apply damage over time i find it best to place him in the front lines as the passive does "stacks" and the "surviving" units will have low hp to be "picked" by next line of defense - for exmaple "Executioner"
active - "Make It Hell" is the perfect counter to any "Heal" effect, make sure you use it when there are lot of healers deployed on the field.