Death-Dealing Dance has two components, a 'swift sword' and a 'striking' component.
- Target attack routine doubled - 20
- 1 Willing creature x1
- Self x0.75
- Duration special x0.6 (see below)
- No save x1 = 9
- Increase weapon damage by 1d6
- 30 Target 1 creature x1
- Range self x0.5
- Duration special x0.6
- No save x1 = 9
- Total = 18
The death-dealing dance's duration lasts until there are no visible enemies or 10 minutes elapses. A typical fight lasts 3 combat rounds. The power cannot be used to "buff" before combat because it would immediately end, and it cannot be used in multiple fights in the same turn (by e.g. rushing room to room) because it ends as soon as the first fight ends. So I rate this as a duration modifier of x0.6. 1 turn is 0.7 for a transmogrification spell and 0.75 for a movement spell, so this is a slight discount to reflect the fact it doesn't actually last for a turn. So death-dealing dance is functionally a single 2nd level spell.
According to the rules in Player's Companion, "Spell-like abilities other than detection and divination should generally not appear as a class power at a level of experience lower than the spell’s level." Death-Dealing Dance becomes available at 3rd level.
Also according to the Player's Companion, a 2nd level spell that is available once per 8 hours with a casting time of 1 round is a single class power. I made it available once per day with an instantaneous use, balanced by the fact that you can't use "buff" with it in advance of combat.
I've run a campaign featuring a warmistress who has 18 DEX and 18 CHA - meaning she's as bad-ass as a warmistress can be. She has not been overpowered. Once per day, she blossoms like a cherry tree of death and takes out a room of low-level bad guys. But is that OP? A 1st level spellcaster can do that too! The rest of the time, the warmistress is a fighter with fewer hp and lower AC than the other melee combatants. In my campaign our warmistress has suffered more mortal wounds than any other character in the game.
Let's compare a 7th level bladedancer to a 7th level warmistress. They have the same hp and armor class. At 7th level, the warmistress has a +3 damage bonus and can death deal twice per day. When she death-deals she will have 2 attacks per round dealing 2d6+3 damage.
At 7th level, a bladedancer can cast 2 1st, 2 2nd, 2 3rd, 2 4th, and 1 5th level spells per day. In an Eldritch campaign where she'd meet a warmistress, she'd be a ceremonialist with trinkets. She could cast Bane-rune (1st level), Swift Sword (2nd level), Invulnerability to Evil (3rd level), and Giant strength (4th level) twice per day, thereby gaining +2 attack/damage, two attacks per round, double damage, and invulnerability to the enemies she's fighting. The bane-rune puts her on par with the warmistress's attack throw.
Is the warmistress overpowered compared to the bladedancer? The bladedancer has to use her trinkets before the fight begins, but the spell durations will be long enough to cover multiple encounters. The bladedancer's damage bonus is only +2 instead of the warmistress's +3, and she might get the bane-rune wrong. On the other hand, she's invulnerable! The warmistress deals an extra d6 damage, but the bladedancer deals double damage, so if she's using a pole arm that will be 2d10 rather than the warmistress's 1d10+1d6, effectively a +2 damage bonus. (If the warmistress is using swords it's even worse for her.)
Not to mention that on top of the above, the bladedancer has the flexibility to use her magic for other purposes, and still has a 5th level nuke.
So it does not seem to me that the warmistress is overpowered relative to the bladedancer, it's closest equivalent.
If you feel the warmistress is overpowered, ban the class! Every campaign is a law unto itself. For instance, if your campaign tends to resolve around single-encounter "alpha strikes" then the warmistress will be noticeably stronger than one where the fights tend to be a series of numerous engagements.
If you want to try to adjust the warmistress, then require her to spend one round activating her dance (similar to how a berserker must spend one round entering greater berserk.) Allow her to use the dance once per 8 hours instead of once per day in compensation.