NOTE: skills listed are those important ones. all of my monsters have 8 skills.
my monsters
level 14 divinegon+10 with vivify, healmore
level 25 kingslime+10 with Increase, EvilSlash
level 20 rainhawk+10 with SpeedUp, healus

starting, my kingslime gets killed lots of times.
after i managed to kill off the 2 monsters, leaving that metabble, which is hard to kill, my kingslime can survive, with heals.
i set divinegon at cautious
kingslime at charge and rainhawk at mixed.
divinegon uses heals when needed, kingslime uses evilslash and rainhawk speedup.

strangely, i found that when using charge, and the enemy is a metal slime type, my monsters will use evilslash.
soon enough, all my monsters speed catches up with the metabble. and my evilslash finally hits! 1 hit ko.

now, my rainhawk lv28
atk 344
def 359
agl 256
int 255
hp 341
mp 359
speedup scorching icestorm paralyze panicall eerirlite hustle surge
divinegon lv15
atk 235
def 230
agl 197
int 192
hp 242
mp 230
healmore blazeair iceair antidote stopspell paralyze fireslash vivify
kingslime lv29
atk 274
def 256
agl 295
int 247
hp 280
mp 250
increase healall scorching radiant revive evilslash barrier healusall

need help to defeat starry night with the least level. still cant make it.

i have a lavaman and iceman in farm, at lv 29, should i mate them and train goldslime?

Yes, just keep breeding your strongest monsters with the ones who have skills you want. The more skills the parents have, the more skills the offspring will have, and they'll continue to get stronger the more you breed them.

I would suggest trying to get a Goldslime for sure. Divinegon and Rainhawk are very good, but you need them to have better movesets, and when you're out you want to use their offensive skills a lot so they'll use them more frequently when you just command them all to attack in battle.

Also having one that heals is always useful, especially if you have it well trained enough to heal of its own accord in battle.