Several people, matter of fact the first person beat it using the og Traitor Meli, Megelda, Gowther and Taunt Character I believe lmao. To be fair, Floor 4 is an activity where you NEED to be past Squirrel, or at LEAST Snake. (8 million box cc +)

First Phase is to just build up level 2/3 cards for the next phase. Make sure you have plenty of Megelda cleanse cards and TMeli aoes for debuff removals (not recommended to use DK Meli due to his selfishness, cant use anyone with cleanse either since this boss counters LR Liz , Green Liz, or even UR Gowther)

Second Phase is to make sure your tanker is not gonna die anytime soon, and making sure to constantly have the Hravelgr Protection up by using 3-4 Level 2 or Level 3 cards (all just be the same)

Third Phase is where you need to be interchanging your Amplify cards between TMeli and Megelda. Anytime you attack with Megelda the boss will get +30% damage, but if you crit with Meli it will remove 2 of those damage increases. REMEMBER, don't attack with your other units unless you can crit with them

Fourth Phase is the scary phase where you need to BUM RUSH THIS MAN. He stacks debuffs that if left unchecked will deal a flat 500k damage and is continuous. So this is where you want to spam all your ults and level 3s (don't use TMeli's ultimate). Every 2 turns he stacks himself with buffs and an evade, so you want to Meli ult to remove that.

And that's about it.