Monster Sanctuary

Story related achievement, cannot be missed.

To those having trouble in the last two battles (Marduk and the Champion), here's the strategy I used.

I opened the battle with L Frosty, L Ice Blob and D Yowie. Spammed Ice shield with both Frosty and Ice Blob, and Ice storm with Yowie, to stack the Chill debuffs on the enemy team as quickly as possible. The enemy monsters went from dangerous to annoying to mild inconvinence as they didn't have enough Mana to keep up the battle (they had 5 stacks each). Congeal did the rest.
The three enemy monsters died at the same time, and Marduk sent his three dragons. Same strategy - Protect with the Blob and Frosty while Chilling the enemy, Ice storm with Yowie. Mid-battle I swapped Yowie with Spectral Wolf for two additional Chill stacks and super-effective Physical attacks. The dragons went down easily.

After Marduk, you'll have to fight a Champion. You won't have an opportunity to swap monsters or change equips, so you'll need to fight him using the same team you defeated Marduk with.
I went for the same strategy (stack Chill debuffs, let Congeal do its thing), and it worked - sort of. Even with seven Chill stacks, he was still able to perform two out of three attacks, and he has a pesky Debuff resistance. I kept on the defensive, keeping shields working and trying to survive as well as I could. I briefly tagged out Yowie for Spectral Wolf for the two additional Chill stacks, then went with Yowie-Ice Blob-G'rulu for the rest of the fight. G'rulu is both a tank and a healer, but more importantly, he can remove buffs from the enemy. As I kept removing the Mad Lord's offensive buffs, he wasn't able to pierce my shields and his health slowly whitteled down to zero thanks to Congeal. I got a 4-star rating at the end.

Equips and food are important, so don't forget those. I went for a mostly defensive build, and weapons with useful passives over high damage output - Restoring Wand, Hexing Rod, Fishing Rod. I haven't included any champion monster in my team to prove that you don't need one to win these fights. Spectral Wolf can be easily swapped out for any other monster with chill stacking if you happened to choose a different partner at the beginning of the game.

Other useful tips:
- Merchants in this game will sell you more items as you rank up as a Monster Keeper. To do so, you'll have to defeat the Champion monsters hiding all over the place in the Sanctuary. Some are story related, but a lot more are optional. Defeating them (and ranking up your Keeper level) is fundamental if you want to be able to buy enough materials to upgrade your equip items.
- Farming money in the end game is quick and easy. Just hatch three Aurumtails from the Snowy Peaks (teleport to the area, then go right and up). They have four "Gold Sense" passive abilities each, which will net you 10% more gold at the end of a fight. Yes, they stack. Use Level Badges (you can buy them from the merchant near the evolution tree in the Ancient Woods) to level them up quickly, then spend their skill points in Gold Sense, Glitter, Gale and every stat/offensive buff they have. After that, teleport to the Sun Palace and slaughter every enemy you can find. Not only will they go down super quickly (netting you more gold-per-minute than fighting end-game monsters), but the chanses of finding a Tanuki in the Sun Palace are five times those in any other area of the Sanctuary. Hunting down Tanukis for thousands of gold each in this place is a lot quicker and easier than farming the end game monsters in the Abandoned Tower, unless you need their drops.
- Feel free to experiment! With three Aurumtails to farm money with and infinite Level Badges purchasable from merchants, testing builds is easy and painless. Every monster in the game is viable as long as you build a good team around it. Don't use random monsters that you think are cool: Have a clear strategy in mind and build your team to achieve your win condition. For me, it was Chill stacking.
- About team building: Be sure to include at least a tank and a healer in your team. Having someone able to clean debuffs from your team and remove buffs from the enemy is not mandatory, but highly recommended, especially in certain fights.
- About skills: Max-leveling every active skill is not worth it. Try focusing on one or two, the most important ones for your strategy, and then go for useful passives which will help you achieve your win condition.

If you’re struggling with this even with Frosty, Ice Blob, and Yowie you can use this team for the entire final act and almost every 5 star challenge.

(L) Caraglow (L) Taragoat (L) Brutus

Make sure all three characters are level 40 for ultimates.

My equipment and food is far from optimized, so if you have better or a bit worse and need to switch it up, I think you’ll be safe. In case you need some pointers, here’s my setups:

(L) Caraglow
Staff +5
Ocarina +5
Scroll +4
Drum +4
Mango, Carrot, Banana

(L) Taragoat
Morning Star +5
Hide +5
Bracelet +5
Gauntlet +5
Peanut, Peanut, Peanut

(L) Brutus
Shuriken +5
Cape +5
Tome +5
Ribbon +5
Pear, Orange, Grapes

Start turn 1 with Caraglow and use Full Heal onto Brutus. This will give him a few buffs and shield.

Use Power Focus on Brutus in this same turn.

Open turn 2 with Taragoat and use Full Protect
Next use Caraglow and use Healing Wave or Restore if you’ve been Debuffed.
Finally have Brutus use Ram Charge. This will almost always wipe the enemies’ starters and make cleaning up the fight incredibly easy. Just keep using full protect and power focus at the end of your turn, and attack with Brutus as the end of every turn after he has Power Focused. Your Flurry Of Blows will usually clean up the second wave.

Here are the skill trees I used
https://m.imgur.com/DInEbMW
https://m.imgur.com/eel6VMu
https://m.imgur.com/bOG5Nh1