Zelgius

Zelgius is not going to double most units in the meta game right now so it would be wasted on him and if you really want to run Desperation on him I would suggest getting a +spd Zelgius that he reaches 36 spd and with a spd seal 39 spd at that point it would be worth running desperation. Only units with a higher spd of 38 should be running desperation. I would suggest on that Zelgius running in the B slot Vantage and then as a Seal QR 3 or QR3 in the b slot and keep the Heavy Blade seal.

Desperation also has more synergy with Life and Death and Fury (Player oriented skills) , than Fierce Stance for the A Slot. You may also want to try the Brash Assault Seal. A sample build would be
(A) - Fury 3/Death Blow
(B) - Desperation
(C) Threaten Def/ Filler
(S) Brash Assault
You also may find much better success with Bold Fighter, but thats unfortunately a very exclusive skill right now, but its super busted for Zelgius/Black Knight