minimum requirement
You need enough END/DEF to survive the Swallow Attack and be able to heal most or all damage of this attack with "bind up". STR is most likely the important stats to "struggle" against these kind of attacks. I recommend DEF being 20+ with a very high STR Build (12+) and DEF 25-27 for heros with rather low STR.
Best Items: a) Peaceful Connection-Totem (+1 CHA / +3 DEF), b) Moos Braclet (+3 DEF), c) Bizarre Acient Ring (+1 INT / +2 DEF) d) any item that raises END Edit: I don't like Murphy Hand, but feel free to use it and destroy the Boss even faster.
Builds: (all stats are BASE without item-modifications, while ATK,DEF,MATK include items)
Warrior (level 20) : 13/7/3/8/4, ATK: 104 , DEF 25 , MATK: 39 With 13 STR my "Warrior"-Eyvind easily broke free with "struggle" immediatly, reducing the incoming damage quite a lot. He hit the enemy for 90+ Damage per Sword Attack. I only needed my own healing spell and the soup to refill HP and MP.
Bard (level 20): 6/4/6/10/9, ATK 80 , DEF 27, MATK 48 With terrible STR 6 "Bard"-Eyvind needed 3-4 attempt to struggle free. However with 260 HP and an insane DEF of 27, Eyvind could easily bind himself up after every Swallow Attack. I needed two fights to beat the slim-boss. On my first attempt the Slim-Boss swallowed me again and again, forcing my to spend 4x4 turns being swalloed, barely binding up until my MP were completly drained - very unlucky! In the End Bard-Eyvind needed a little bit of luck, soup and two MP potions.
So the player got two options to beat the slime: extremly high STR or very high END/DEF.