I'm not looking to have her be the biggest, toughest of them all. I'm looking to have her be what she should be according to her class and level. Ragzlin is a hobgoblin Warlord, or barbarian as you say. Either way, he should not be too hard to hit because he is a damage dealing enemy, meaning his purpose is not defense but offense. So I would expect him to hit hard but to be easier to hit. Gut is a magic user, possibly another cleric, but more like a goblin shaman, so I would expect her to be a bit squishier but have more spells. Both of these two seem to be built just fine with these concepts in mind.
But Minthara is a full-blown cleric. Clerics are tanks, meaning they have high defense and lower offense, typically with healing spells and spells that buff allied stats. With her AC being so low she is the antithesis of what a cleric should be, a tank.
So I'm not looking for her to be some really tough damage wielding offensive and defensive Superior boss. She should have high AC and do a lot of buffing her minions. Again, if Tav and several party members can have, by that point, an AC of 20 or more, it isn't unreasonable for a cleric boss to also have an AC of 20 or more. Again, I'm just looking for her to be a match to the players, not overpowered. If she has a base AC of 15 due to her armor, that is a basic AC for clerics, not even enhanced AC. If you're equipped with half plate armor like Lae'zel starts with, and you add a shield to that, your character would be at AC 20. Why should Minthara, then, be in some sort of armor that is inferior to your and your companions starting armor? You're starting armor can be, and often is, better than minthara's special drow armor.
Again, they have her suited up as if she is wearing a rogues armor. Not a cleric, but a rogue. I don't think what I'm asking is some super huge deal because I'm just asking for her to wear armor that is appropriate for a drow cleric at level five. Her armor should be no less then 15 or 16 plus dex.