Abilities

Protect 
Clash: +6 Shield to 3 nearest allies
Max 10 Shield, 4 allies

Protect 
Clash: +6 Shield to 3 nearest allies
Max 10 Shield, 4 allies
+2 Shield
Lvl 1+1 target
Lvl 6+2 Shield
Lvl 11

Guard 
Deal 2 DMG to adjacent enemies if allies lose Shield
Max 6 DMG, +1 ATK

Guard 
Deal 2 DMG to adjacent enemies if allies lose Shield
Max 6 DMG, +1 ATK
+1 DMG
Lvl 2+1 DMG
Lvl 7Grand Warden +1 ATK for 6s (stackable)
Lvl 12

Brave 
Shielded allies +5% Hit Speed
Max +20% Hit Speed

Brave 
Shielded allies +5% Hit Speed
Max +20% Hit Speed
+5% Hit Speed
Lvl 3+5% Hit Speed
Lvl 8+5% Hit Speed
Lvl 13
Stats
HP 10
DPS 0.6
Attack 1
Hit Speed 0.6
Energy Cost 0
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 enemies when his allies lose their shields. So, melee units will take damage a lot harder than ranged.