Boss need to be that powerful because defensive skills are just as power as the boss skills. Every type of bosses will have a certain counter to them you can use and completely prevent them from doing much unless it's a really long and exhausting fight one like Adrablahlbahlah (not gonna spoil the name)

If they are physical attackers (Looks like a monster or fighter) : Use Evasive aura, source orb in a necklace slot of Huntsman source book comined with Aero source book. Hunts 2 aero 2 to learn it.

If they are magicians, Leadership and resist potion before you trigger the fight.

If they are summoners, AoE source skill + source vampirism and maybe eat some source orb and source potion if you want to be able to use another 3-source skill after it comes off CD to destroy the boss.

The only boss that breaks this rule so far is sadly only level 14 and can easily be overpowered with gear along with a few fights that have bosses of multiple type instead which are very and keep you on your feet because of the stream of attack that comes on each turn and with round robin, you are given the turn to react after each set of attacks.