To be more specific, they barely spawn at all when grouped except on the boss. When solo, they spawn in pretty much every area except the Howlitzer Gates and Coliseum. This makes soloing HoI much, much harder than doing it in a group. I don't care which way it's fixed but make them more equal at least.

On the topic of Frozen Souls in general, I'm of the opinion that they either shouldn't explode (and cause damage/freeze) if you destroy them yourself, or they shouldn't be able to spawn on you and immediately start shaking. Alternatively, I'd take the explosion on player hit (as opposed to self destruct) being much smaller.