If Irenicus is level 30, he'd wipe the floor with Severok. There are so many different tactics he could use. One thing he could do is simply gate in high level monsters, keeping himself hasted and invisible.
And how about Tensor's Transformation? How would the battle go if the first thing Irenicus cast was a host of defensive spells (Blur, Mirror Image, Stoneskin, Simulacrum, Improved Mantle, etc...) and then cast Tensor's Transformation. And what about Time Stop? Just because Severok is immune to magic doesn't mean he's immune to time being stopped...
I think the clear winner here is Irencius...easy. At level 30 he has enough magic to use pretty much any tactic.
Glycerine