So after reading the responses more thoroughly, you've once again missed the point.

Let's just take one boss. Ragzlin. Should he be tougher than the Hag? Yes! Why? HE'S ONE OF THE STORYLINE FINAL BOSSES. So when I face Ragzlin, I should be facing a beefy boss guy. It should be a difficult fight for your characters.

Right now, we're capped at 4, but later we won't and we'll be at level 5. Hobgoblin Warlord is a CR 6. So, a party of 4 level 6 characters could relatively beat him without much loss. Therefore, using D&D DM game campaign design, if I want a boss fight for 4 level 5 characters to be challenging, I would pick a boss at a CR slightly higher than my characters average level. Thus, a Hobgoblin Warlord works VERY well for a boss fight for 4 Level 4 or 5 characters.

And as for Kagha/Olodan, YOU ALREADY GET HELP DURING THE FIGHT. So the ONLY thing Im suggesting is that Larian beef their stats and make them appropriate and then balance the fight from there. If they have to add a few more characters on either side to balance, then what's the big deal?