Forcemaster's biggest weakness against a Warlock is direct damage and debuffs.
Instead of getting in a boxing match with her up front, bury her in debuffs first and save the boxing for when she is too busy trying to run damage control.
Use a Wall of Bone (or wall of fire, but bone is cheaper and lower spell cost) to block LOS from her Battleforge.
Then Mage Bane. Then Ghoul Rot. Then Agony. Give her an Enfeeble so you can safely toss fire spells from the distance. Have Moloch's Torment handy for extra damage from your curses. She has to decide if she's going to purge these buffs or just deal with them. If she doesn't purge them she's going to take 3-5 damage every round that bypasses forcefield or defenses.
As she struggles with debuffs and direct damage, she is either going to have to go all out and just try to deal with it (which will let you just play defensively while she melts) or will have to reduce her aggression to try to keep purging them. if you want to keep distance, tangle vine her in place and toss Fireblasts (unavoidable, bypasses defense).
As your anti-forcemaster creature stable use:
Gray Wraith and Skeletal Knights or Sentries (Mort is good to because he can get some taint tokens in there). They are Psy-Immune so she cannot put them to sleep, charm or mind control them.
Another good tool is to give yourself a damage barrier. Any of them are good. The fire one will make her burn herself for hitting you, the lightning one will make her stun/daze herself, or Demonhide Armor will give you some armor too.
In general you want the Forcemaster overwhelmed with things she has to do. "Do I purge debuffs, remove your buffs, manage creatures or attack?" She is the mage least capable of handling multi-tasking, and is the most predictable mage in the game.
Those are kinda general tips you might try to work around or tinker with. The Warlocks biggest advantage over the Forcemaster is his overall flexibility compared to her very rigid strategy. As a Warlock you can shift gears between taking your time, or being aggressive, using creatures, fighting at range, debuffing and direct damage, theres a lot of options you can work with to answer her much more limited approach.
