Another issue with Highsun, though secondary to the problem of it being unresistable, is that it stuns for 1 second instead of 0.001s.
Many spells that have a stun component (e.g., enchanter nukes) historically all had stun durations of 1ms until late 2004:
2004-12-15 10:29 Changed Slot 3 from "Stun(0.00 sec)" to "Stun(1.00 sec)"
The Velious spdat for Highsun shows its stun attribute as having a value of 1ms. The p99 spell file also shows the same. But on p99, any spell with a stun value <1000ms (this includes mez) stuns for a full second regardless. Is this intentional?