Tempered in Flame (Deadly) is the only stage in XP Colosseum.
The level starts with a 200% Kurosawah spawning, and soon 1000% Doge Darks in groups of 3 appear periodically. After some time, 3 Bun Bun Blacks 200% will also spawn. If the player takes too long (about 4.5 minutes) 100% Tackeys will appear, and will be constantly spawned until the end of the game.
Managing money at the beginning of the stage is rather difficult since Doge Darks can easily rush and destroy the base within 6 hits.
CPU Cat can be useful for meatshielding, however it may be not too useful until Bun Bun Blacks appear. Sniper isn't nearly as useful as the strong enemies come in numbers. Speed Up is also not that impactful (feel free to use though) as the stage is quite fast-paced.
Cat Jobs will earn more XP for beating this stage, so if you can beat this stage reliably, you can get the most out of your Cat Jobs power ups here.
Treasure Radars can be used to guarantee the 1.23 million XP treasure drop. Buying Treasure Radars from the store and beating this stage is the most cost effective way to spend Cat Food on gaining XP. If you're confident in your lineup, you can even bring Freshman Cat Jobs in order to increase the XP gained.
Lineup: Two freeze combos, Awakened Bahamut (30), Manic Mohawk Cat (30), Manic Eraser Cat (20 can work, 30 recommended), Bombercat (25-30), Manic Lion (20/30), Manic Flying/Jizo or any other cheap DPS unit.
There are many other ways to do this, if you don't have freeze up, you can use a research up combo. you don't even need a combo if you have good bomber RNG.
At the beginning spawn Manic Mohawk first off to reset Kurosawah's attack and spam Manic Mohawk, Manic Eraser, Bombercat and Manic Lion to perma-freeze Kurosawah and the Doge Darks. When the Doge Darks die, you should have enough money to spawn Awakened Bahamut, keep spamming everything except combo units and you should win. Enjoy your truck load of XP!
Battle Items: Cat Jobs (Recommended), Treasure Radar (Optional)
Remember to immediately spawn two Crazed Cat, and then a bomber. This is extremely important if you don't do this you will likely not be able to do the rest of the fight. As soon as you enter the fight spawn crazed cat, another crazed cat and then bomber cat. Stall with Crazed Cat, Crazed Wall, and Bombercat after the opener. The moment you start to kill some of the doge, use wall cat as well. when you have enough money, spawn Awakened Bahamut. After you spawn Bahumut, Bun Bun Blacks will spawn. You should have no problem defeating them by spamming meatshields, Bomber, and Crazed/Manic Legs.
The fight is over. For more info watch the YouTube Video linked in below. It has two attempts and some important comments to read.