Heirs of the Kings

Lucifer is the optional and strongest boss fight of the game. You find him in the Envy Shrine (left of Zarpatt Town) inside a purple warp zone. Here you will also beat the trials (small bosses along the way) to collect holy weapons. You don't have to use them for this achievemtn. Beating him is the only thing you have to do.

I kept my tactic very simple, i used Final Strike with Grant and Armor Break with Serena in combination for a heavy damage output + Revive with Laura if needed and Shadow Bind with Bernard to slow the boss down. Keep in mind that's important to upgrade skills like Final Strike in the soul map section.

I had to grind KHP to be able to buy many Ability Seeds for Grant. 10 KHP gives you 5 on all base stats and all base stats can raise up to a maximum of 999. You can also modify your weapons with STR buff to reach that cap faster. To speed up the grind buy a monster attract ring and you have an encounter with each step. The other important thing is to wear a critical ring (listed in the shop) for even more damage output.

I can't hurt to start a test attempt and if you still feel too weak go back to KHP farming and give it another shot later. Good luck!

Lucifer is final boss of a dungeon that appears after the normal ending called "Envy Shrine".

This dungeon is located to the left of Zarpatt town surrounded by mountains. (Purple Portal)

The dungeon is very linear and you will fight 4 sub-bosses which will drop a God weapon for each character. God armors are available also if you explore a bit in treasure chests. You can use these weapon/armor or not.

To prepare for the Lucifer fight, I did this. Grinded a lot of KHP to buy about 15 Ultimate Seeds to max out his STR to 999. (You can save, use an Ultimate Seed, if you don't get +50 to STR, load, repeat).

Unlock his Final Strike skill in the Soul Map and use Reset Stone to upgrade it to level 20. (I suppose you can go further, but level 20 was good enough).

I also upgraded Armor Break to level 2 for Serena and Shadow Bind to level 2 for Bernard. This is probably not needed, but it was the main skill I was going to use.

I also went into the Lucifer fight with about 800 KHP. Just unleash, Final Strike, Armor Break, Shadow Bind every turn. If everyone dies, use 50 KHP to come back with full HP/MP. I was able to kill Lucifer after spending 200 KHP. Depending on if you are stronger/weaker than my party, it might take more or less, but if you are around level 65 or so, and have prepared, I think it should be no problem to beat him within 3-5 full restores.

Good luck!