Each 3rd level imbuement has a BOSS / QUEST you have to do:
- LLOYD (thais energy portal):
- Energy Protection
- Energy Damage
- Speed
- LADY TENEBRIS (thais death portal):
- Death Protection
- Death Damage
- Life Leech
- MELTING FROZEN HORROR (thais ice portal):
- Ice Protection
- Ice Damage
- Shielding Skill
- THORN KNIGHT (thais earth portal):
- Earth Protection
- Earth damage
- Sword / Axe / Club Skills
- DRAGONKING ZYRTARCH (thais fire portal):
- Fire Protection
- Fire Damage
- Mana Leech
- TIME GUARDIAN (thais holy portal):
- Holy Protection
- Distance Skill
- LAST LORE KEEPER (thais final boss):
- WORLD DEVOURER (heart of destruction final boss):
- WARZONE 4,5,6:
- NIGHTMARE BEAST (dream courts final boss):
Now, back to your question, in my opinion The Last Lore Keeper is the hardest boss in the game to be completed. Because it requires HIGH coordination from 15 players, so usually the team will just run out of time and get kicked, and is also a lot of damage due to the summoned swords.
So, in order to unlock critical, I'd definitely go for World Devourer instead. If the paladin is a skilled player and with the right equipment, he can easily be "carried" by a higher leveled team. I'd say even a 200 RP could do it. All he has to do is, in the final room, stack behind all other players, so he won't get directly targeted by the creatures in the room. With might rings he can reduce a lot of the wave damages, also switch to energy ring if HP goes down, it is doable.