Update:
After observing quite a while how bots level up in open fields, Ive seen that they are using a hack similar to "Cabal's No Cooldown Hack".
In which they can cast all skills at once without having issue with Mana or Cooldown.
This is an old video but this will give you insight of what Im talking about.
Not MU btw.
See the mage (new character created) where you see 1 skill but with A LOT of misses (since the target died already.)
Thats how they do this in MU Legends.
Now, imagine this scenario:
You are in dungeon, you see the Grey-Rainbow Shine (orb) (double damage buff) and you use all your skills at once.
What do you think will happen to the boss?