Abilities
Passive
Super: Grand Warden and his 3 nearest allies gain 12 shield
Promotion 1
Shielded allies gain Rage
Promotion 2
When allies lose shield, deal 10 damage to surrounding opponents
Promotion 3
When allies lose shield, gain a stack of +1 attack damage (max +8)
Stats
Classes
Mage
Grand Warden Counters
Grand Warden's main purpose is to buff its team. So rather than specifically countering him, you need to counter the Minis he buffs. Having units that deal more damage to shields or slow opponents will be very effective.
To take out Grand Warden himself, ranged troops that can reach the backline will be great for dealing damage earlier in the round. This is important because he deals extra damage to adjacent opponents when his allies lose their shields. So, melee units will take damage a lot harder than ranged.