If you're being one-shotted at Level 5, you might want to check your equipment. Even if you've got pretty low Toughness you shouldn't be taking that much damage in a single action unless you're getting super-unlucky with the RNG.
Another fairly obvious question but it bears asking is: Did you go into the Alissa battle with reduced health from the previous encounters? Because if so, you should consider stocking health and resolve-restoring items and making sure you're more or less topped up before the fight.
Assuming neither of these things is the issue, there's a couple other things you can do to help you with this fight. Depending on who your companions are, there are multiple ways to reduce the damage you're taking even independent of your own equipped powers or reduce your chances of getting hit in the first place. If Atugia's in your party she should be drawing most of Alissa's attention, Cait can use Spirit Veil on you, Ryn can get some well-earned payback by shooting her in the face with Aurora Arrow (giving your whole party a nice defensive buff while also hitting hard), Kiyoko has several defensive powers in her Kami-no-Michi set and so on. Switch to Manual input and make sure some defensive buff is applied posthaste and that should help a lot. Also, you can down some Spiced Wine before going into the dungeon if you need a Toughness boost, just don't take a drink once you enter the palace because the Winterberry Wine effect will overwrite the Spiced Wine one.
Alissa has the same damage-boosting Perk that a Thief gets which means that any time you or a companion is suffering from a status effect, you're going to take a lot more damage from Alissa's attacks. Having a way to clear those effects can help if you're dying quickly. Also, having a good source of healing is a pretty good idea for this fight. If you don't have Cait in your party Ryn can do an even better job of it with her shiny new gear, which also nets her Spirit Veil for another defensive buff option.