How to build Ishtar without desperation?

I'd honestly just wait for Desperation fodder to appear. Normally, Desperation is put on units that don't even hit that amount of speed required to double most enemies, but this is one of the rare times when Desperation actually works well with the unit. Ishtar hits 46 speed with her tome and Swift Sparrow active, and that's more than enough to double most enemies. I've seen people suggest Desperation on units that don't even hit 40 speed with their skills active, and yeeeeaaaah, it's not very good.

You can never go wrong with Swordbreaker or Lancebreaker until you get Desperation.

Honestly, whenever I have no idea what to put on the B-Slot of a unit, or I don't have a the skill I want to give, I just give it a -Breaker skill to guarantee that my unit can put in work.

Her durability absolutely implies she doesn't want to get hit...you can maybe try Watersweep and at least stay safe from Dragons and enemies who can't counter you. You're going to want to maximise on Atk as much as you can so you can oneshot consistently.