I played with these 15 codes:
W1A7W101 - Dimensionhole
6B4Z61EX - Wasteland
73153736 - Slate Warrior
QX2DGFCM - Muka Muka
2FU7AFLF - Yami
4P0HAD6X - Dharma Cannon
8B6PN5JK - Cannon Soldier
Q9QV4BGX - Barrel Rock
0KAB65EK - Riryoku
8F2B29G5 - Dark Elf
SXJXCR2R - Cosmo Queen
GXN767YK - Cosmo Queen's Prayer
G3CD87ND - Princess of Tsurugi
47EBLHA7 - Dragon Zombie
SPUHNMU9 - Violet Crystal

Removed basically everything level 4 and above

- Lab breakers not as useful as I expected despite labyrinth everywhere
- Slate warrior actually better than vanilla
- Muka Muka showing up of course
- Cosmo Queen is OP but a bit clunky
- Wasteland not getting much use, but I didn't code in mammoth graveyard

I think these would be worth putting in:
YBJMCD6Z - Greenkappa
UFYKYZYT - Eyearmor

Most problematic duels: Bonz, Pegasus, Final boss

Update: first timed run 2:09:36
Square x5, L1, 
Square x2, Down, Square x2
L1, Up, Square, Down, Square x3
L1 x3, Square, Down, Square

W1A7W101 - Dimensionhole
6B4Z61EX - Wasteland
73153736 - Slate Warrior
QX2DGFCM - Muka Muka
2FU7AFLF - Yami
4P0HAD6X - Dharma Cannon
0KAB65EK - Riryoku
8F2B29G5 - Dark Elf
SXJXCR2R - Cosmo Queen
GXN767YK - Cosmo Queen's Prayer
G3CD87ND - Princess of Tsurugi
47EBLHA7 - Dragon Zombie
92886423 - Goblin Fan
YBJMCD6Z - Greenkappa
UFYKYZYT - Eyearmor
SPUHNMU9 - Violet Crystal

Ishtar was a bigger problem this time, and i even died to panik a couple times