Phew! I just landed a hail mary and 3 mickey'd Lexaeus 12 and 13. Big thanks to my Guild Oblivion peeps for the guidance as I tinkered with my loadouts.
Lex 12
This was by far the hardest to figure out, and I had to spend Jewels to 2-turn it. Alas, I got it done after a lot of trial and error.
DR 29
Sebastian, Sp 1
Key Art 5, Sp 1
iSora2
Moana, 92%
Xigbar B, AB3M, 100%
Friend: KDG, AB3M, 150%
Initially I was trying to get SC to proc from a friend medal, but that was annoyingly unreliable and it stifled my DPT ability. I landed all 6 hits on both turns, so that was lucky RNG. I died on the enemy's turn 1 attack and hit continue. This started T2 with full HP/Gauges and no paralysis. I messed up the first time I did this by continuing on a mid T2 countdown attack, rather than from the T1 end attack, which cost me 100 extra Jewels.
Lex 13
DR 29
iKairi2, DB2, 114%
Pluto B
Key Art 7
Boosted Classic Donald, DB3M, 100%, +1k ATK
Pluto B, DB2
Friend: iKairi 2, DB3, 150%
The boss fight lasted 15+ turns, but it was easy once I figured out the tank route. I initially tired more damage focused setups, but simply couldn't survive. Once I heard about Pluto B's usefulness I changed my strategy. DB3 or SC is needed to survive the second monster's preemptive attack and I don't have either, so the friend slot had to be the source. DB3 won out because it's repeated procs are far more useful in a long fight like this one. The dual Pluto Bs are used to survive in the event that the DBs don't proc vs the boss.
Final Result
Lexaeus B, 89%, +1k ATK
Feels like it was well worth the 200 Jewels. I may drop 11 mirrors on him, too.