Yeah, that's on me - I misunderstood the way her Mad Assault ability works, and I've edited the guide to reflect that. The actual tooltip reads, "Deals 2d12 damage when used with the last Mana Point and Player Damage equals 0." I originally thought that meant, "player has taken no damage," but I was wrong - it means, using it when you've dealt no other damage in the turn.
In other words, if you use it when your Damage rating for the round still reads 0, and you have only one Mana to spend, you'll do 2d12 instead of 1d4... which means you can't use other damage-dealing abilities in the round if you want to trigger that extra damage from Mad Assault. Makes it a bit tricky to use.