OR
If you have a high-level thief (Jan comes to mind) you could lay some spike traps near the place she spawns. 3 is just enough for her to get hit real bad, plus she won't heal herself which she will do if you go for all-out brute force. This tactic does sound cheesy, but it works

I've yet to defeat her without Jan's spike traps though.
-edit-
Just a thought. Try using Protection 15' Radius on your party and summon Fire Elementals, Solars/Planetars and the works to keep the Slayer shadows busy. Place them at close proximity to where the shadows will be spawned and they will immediately go for the summoned creatures. You will have to work quickly though, since they won't last long under the barrage of slayer shadow level drains. Improve Haste your party and hightail to the area where Amelyssan will shadow-door to and beat the crap out of her (use Greater Whirlwind, lower resistance, pierce magic etc.) You must be quick in this execution or she will attempt to fully heal herself at which stage you might consider reloading.
Also, if you happen to have a Wish scroll or spell, cast it and wish for a rest so that you can rememorise your spells. It can make a real difference.
Hope this helps. Overall a very frustrating battle, but then it IS the final one.....
[This message has been edited by tusitha (edited 10-08-2001).]