Thanks Homeless Joe!
Creature-wise, the Druid has the following creatures which could be "turned"; Vine Snapper, Tataree, Thornlasher, Togorah, and Raptor Vine. Kralathor is Epic, and cannot be turned into a zombie, else he would be a great choice.
Your idea of using the Warlock is very good - all plants have a weakness against Flame and he has a nice advantage in that respect. However, she has a number of ways to deal with Burn - including Renewing Rain and Mohktari's Branch. We've done a lot of tests against a Warlock build. Despite her flame weakness - she is very hard to "finish off". Her Lifebond trait (which is acquired when she uses her Treebond ability), combined with the tree's natural regeneration ability, and her own, makes her an extremely difficult foe to finish off. In playtesting I found myself playing against her a lot just to see how in the world do you take her out!