Agnu the Volcano Lord. It can grant any card the ability Battle Ready, as well as +1AP and -1HP. Costing only 1 red energy, it can be played on the first turn of the game, and used soon after. Careful, though-- if used on an ally with 1HP, you'll accidentally kill your own ally, hurting yourself instead of your opponent.
Strong against: slow-developing decks, milling.
Weak against: stupidity, healing, object removal and ally-heavy decks.