A3 light can do static as well, Act 5 merc wil cast a very low shout.
Id go with A3 aswell
A3 merc will keep distance so he doesnt rly need potions, and if he dies, just revive him

Oh and if u use CTRL+Drink potion, ur merc will drink it, thats the short way, another possibility is dragging a potion onto ur merc portrait and drop it there.