Priests are support chars if they are killing stuff you're doing it all wrong, or you are about to wipe. At most they are a very temporary emergency extra tank, and that usually means you messed up. The idea with a priest is to multiply the effect of other chars via buffs and heals. With a priest behind them a fighter has 1.5-2x more hp and can thus deal that much more damage.... and squishies don't die (or come right back after they do!). Xoti single class is doing great for me @lev 16 PoTD (Trial of Iron)... The only thing she has that's damage useful is a spiffy good hunting bow. All my chars are singles... Eder Fighter (tank, can't hit **** but sure can take a hit). Serafen Barbarian (off tank, dbufs, hits ok) Xoti as mentioned and 2 wizards, Aloth + Main. ... you don't know happiness until you've tried to fight smart and conserve the per rest stuff and a fight starts to get out of control... then you say "enough of that" and lay down Power-Up + Empower + Wilting Wind with two wizards, to leave just one opponent standing at bloodied... After which one can only say "because I said so!"
Two tanks, two wizards and a healer, all single class (plus something for fun like rogue or ranger in games where 6 party members are allowed, don't like the 5 char party setup here at all, makes it boring) is easy mode and owns every one of the BG, POE, and Divinity games and really any thing similar so long as you use terrain (Divinity was particularly good for that with the high ground bonuses) to your advantage religiously, don't neglect crowd control spells and as long as you don't let your wizards be too specialized in one damage type (one key reason to have 2). This works so well that I've been forced into playing on ToI to keep it interesting. (I prefer that to level scaling which is silly since it means there's no reward for playing well, with ToI you're level of care in choosing your battles and not fighting every time is way more realistic ).