What is formless mastery? You're not the one who should complain about overpowered, you get +4 to hit, +4 to AC and +12 to damage
Assuming you pass stunning fist save in round 2 you win
Combat Recap:
Kobold 155HP (or 143 if 6 per level which is used here for balance)
Monk 159HP
R1
Monk Trip (1d20+21)[34] +2 for 36 vs CMD 31 (kobold tripped) 17 damage
Kobold 126/143
Kobold is prone, full attack
(hits with bite (39-4) claw 2 (44-5) for 18 + 24 + 11 = 52 +1 acid damage
Monk passed nauseate DC 26, but is intimidated (needed 21, rolled 34 ==> 3 rounds total)
Monk 106/159
R2
Monk six attacks (shaken, against prone)
28, 25, 39 (25 damage), 36 (29 damage), 39 (36 damage), 27
Kobold: 36/143
Kobold stand up (OA misses), activate bodywrap, stunning bite (hits AC 35) 23 damage
Monk: 83/143 Fort DC 26 (rolled 26 barely managed (you again didn't include shaken penalty) )
Needs to roll DC 22 vs stunning fist (if you fail, you die) <-- this comes before round 3
R3
six attacks again
39 (20 +3), 30 (18 +1), 31 (14+6)
Kobold: -6 on second attack, heals 10 immediately and is dropped again with third attack
Even with 155 HP I'd be at -14 after third attack and healing would prevent insta-kill, but would not keep him in combat.