Astria Ascending

First note, lowering the battle difficulty in game settings does not impede any achievements. Feel free to lower to 'very easy' if you're having trouble or just want to speed things up. The last fight here can still be a struggle even on the easiest setting.

--skip below for the meat and potatoes (Also Update courtesy of EmptyFibers)

tl;dr - Save scum "Steal" vs the final challenge boss for his rare steal "Harmony Ring", repeat x4 for your whole party to be immune to his debuffs.

The arena (Leo Temple) opens rather early in the game, and allows for 5 of the 10 challenges. The first 4 are waves of baby enemies that can mostly be aoe wiped or trading regular attacks. Challenge 5 is significant as a means to grind SP efficiently early on.

Changing to very easy, you can kill him in a few rounds of normal attacks(or even 1 attack with Alek's multiple hit ability) and be rewarded a repeatable 25sp. If you're playing the whole game on very easy, grinding here at all doesn't make much of a difference.

--meat
The remaining 5 challenges open up at post game. Similarly, the first 4 challenges here are waves of baby enemies that you can no doubt delete using aoe black magic. The final challenge, against Amanite, is a rude wake up call for anyone that idled through the game on very easy.

Phase 1 has 56,350hp, weak to fire and light, and is a complete breeze. Just make sure you do not push him into phase 2 with a multi-hit ability as he will retaliate multiple times to match(Using gunner to overkill 6 hits will see him getting 6 free attacks on you when he transforms, a.k.a an immediate loss). Phase 2 is where it heats up.

Phase 2 has 289,550hp, all stats nearly doubled, Loses the fire weakness, but is still weak vs light. He will use single target dark damage or omni triple slash for aoe physical that hits 3 times. The main difficulty lies in his ability omni malediction which inflicts all status ailments on the entire party. However, the fool is hoisted by his own petard.

--potatoes
vv Update vv
This guy has a rare steal "Ring of Harmony" which you might know in other rpg's as a "Ribbon". This makes you immune to all the major ailments except the stat downs and weakness.

The success rate of a normal steal to hit rare is rather low, and you have to continue the fight since there's no escape option. Except there is! Arpajo's cosmo break "Warranty" gives you a 50% chance to grab the rare steal AND escape from battle(which even works in arena). You can quickly fill his bar with 7~8 steals from our old pal Onis in challenge 5. Just remember to set his support ability "First Strike" to expedite the process. Make a save when you're full and use warranty on Amanite. Reload if you got the stinky HP Cape and repeat until you have at least 4 copies.

--dessert
With our team of mostly immune characters, we can enjoy a stress free fight. Wall/Shell aren't so needed since he overwrites these with unwall/unshell. You can counter his malediction with a harnesser's substitue, or just ensure your dps has a Coward guard ring.

My team was Ulan, Eko, Kayden, Alek. Set up unwall claws/physical weakness with kayden and Regular attack down phase 1 with Alek. The other 2 can build up focus or use crystal break to buff up our dps. With the debuffs on (phase 2) A 200% boosted triple slash can do well over half it's health in 1 shot and potentially kill if you land 3 crits.

Achievement unlocked! You also gain a massive 10,000,000exp for your first kill (only 300,000 for repeats q.q)