You're way too underleveled for that encounter. After doing everything you can in that region before that point, your party should be level 8 as well.
Even on explorer this requires perfect execution and an in depth knowledge of all the combat mechanics.
It's exactly because you're playing on Explorer that this encounter were even doable at all with a party three levels lower than the intended level. I don't know about Classic but on Tactician it should be impossible (without some god-tier cheese, anyway).
I feel as the entire purpose of explorer mode was that you don't have to first master the game to do a play through.
More or less, provided you're tackling encounters
of the same level as yours. Or one level higher at most. The very first key to combat is
level difference. One level difference makes a huge difference, because your HP and the magnitude of all spells/skills AND stats of equipment scale strongly with level. On Tactician with solid understanding of everything and perfect execution, with a standard party and decent gear (so no Lone Wolf or any other cheese), an encounter of two levels higher against at least four enemies will be a real challenge but doable (usually).
Back to your situation, you should be level 5 (or close to) just by completely exploring Fort Joy and doing everything you can in there. After that, by exploring the remaining areas and going through several other encounters, you should be able to hit level 8 before taking on Alexandar. If you're still level 5 when getting to Alexandar, then there's a whole lot of stuff you haven't done and explored. There are a bunch of encounters of level 5, 6, and 7 around Fort Joy, and also keep in mind that you get XP just by exploring new areas.