After a bunch more games over the weekend, here's what I found:
Wall of Thorns trick is very effective!
Wizard-
Earth Wizards have the best chance of any mage to take down the Forcemaster. You can save Nullify for Bear Strength. Hurl Boulder can keep her Dazed so her huge attacks and Battle Furies miss. If she sets up the Battle Forge, save your Dissolves for Dancing Scimitar or her wands. Seeking Dispel/Invisible Fists on a stick can really wreck you! If the Forcefield combined with her Deflect is too much to punch through, then set up the Force Push/Wall of Thorns to eliminate the Charges and push her into your waiting Golems... She'll still get 2 cancels (3 with Scimitar) a turn... Or just Dispel it for one more mana than the wall combo. Keep your Golems grouped together, and effective group attack takes a full action and you can respond with Group Heal. It's much easier to Teleport her to your Golems. Most Forcemaster builds farm Battle Forge/Harmonize and Crystals the first two turns, this creates a devastating 3-card hand every round, and since she can play the "react" game and doesn't use mana for creatures she'll win the mana war every time. Don't be aggressive. It may seem like hitting her before she has a chance to build up would be best, but her defenses cost very little mana and her sword is stronger than your attacks. Try Teleporting forward one zone first round and cast Voltari Gate next to you. You can Harmonize and use it to spawn your Golems while you build your defenses and mana. Then you can Teleport her in on her initiative for a raze of attacks, then on your initiative you can Nullify her and follow up with a Tanglevine to prevent her from escaping or Teleporting out.
Fire wizards can use the same strategies unless she equips the Hauberk. You can also try summoning Gorgon Archers if you can keep the Mind Controls off them. Weak is super effective is she doesn't run Purify.
Air Wizards can ditch the Golems and Gate, saving your mana for a very aggressive solo lightning assault. As long as you can Dispel her Forcefields, you can win the attack war with Dazing unavoidable attack spells.
Beastmaster-
Spam the arena with creatures, but spellbind a Force Hammer to deal with Obelisk once you get a few creatures out. Keep your Pet Nullified and watch out for Mass Sleep. She will most likely only be running one, if any. You could try a Force Wave through a Wall of Thorns. Falcons are very effective because they are fast, easy to summon, and work well with the Wall of Thorns combo.
Priestess/Warlord-
Both of these mages are efficient at building a defensive stronghold and spamming range strikes. Use Wall of Thorns to take down the Forcefield and Dissolve her defenses. Ignore Charms and just guard and let her pay the upkeeps. You can summon more creatures. Save Dispels for Mind Control. Eliminate Obelisk ASAP. Late game, the Forcemaster may destroy the walls and decide not to pay the upkeep on Forcefield just to replace it with a new one. If you're out of Dispels, then Guard is your best friend! As she spends attacks taking down your minions, you can punch through her defenses to set up your final attacks.
Warlock-
My versions of the Warlock have all been aggressive builds: Lord of Fire buddy, Werewolf, Solo, etc.
So far, I've had No success against the Forcemaster. Lord of Fire will get Mind Controlled even with a Nullify (She can Seeking Dispel/Mind Control in the same action). Werewolf dies too fast. And I haven't been able to keep up with her doublestrike and defenses when I run solo. There are too many items to Dissolve. Dancing Scimitars and Hauberk and Elemental Cloaks keep wrecking me. Agony gets Dispelled right away. The low mana production of Warlock simply cannot keep up with Forcemaster's arsenal. I tried a slower approach, building up my equipment... but she just put Drain Power on Mage Wand to win the Mind Control/Dispel wars. Besides luck, I haven't found any effective strategy.
For 11 Mana, her sword rolls 8 dice (10 with Bear Strength), while Warlock's 8 mana sword rolls 5.
But it's her innate Deflect that really tips the scales... Free mana 50% miss for my burn spells or attacks.
Hope this helps!