Test Attributes, to be able to try encounter with the concept Warrior:
HP: 1200
Projectile Attack dmg: 10
Jump Attack dmg: 20

- Jumps need a telegraph on the ground.

Slime King
Base Size 800%

100%
Every 4 Seconds Shoots a ring of projectiles and another 2 rings with each 0.2 seconds delayed and slightly shifted.
after 4 repeats of this pattern, jump in the air and target location player currently at. Lands after 3 seconds.
After landing creats 1 ring of projectiles and takes 2 extra seconds to recover.

repeats behaviour until 75% is reached.

75%:
Size changes to 600%
Spawns 2 Slime adds (Monster C from Concept for now)

shoots every second with 3 projectiles (shoots out the projectiles in same moment with 45 degree shiftet flight location) in players direction.
after 5 shoots it charges for 5 seconds after that the king does 3 rapid jumps aiming for the player. 
first should be dodgeable. quite easily.
second needs to be blocked with movement skill or iframe (unless high movespeed, 110% shall not be enough to dodge this, 110% can be achived without any special gear as bard permanently)
third jump is a little slower that should allow a player with 100% movespeed to barely run out of it.

50%:
Size changes to 400%
Spawns 2 Slime adds (Monster C from Concept for now)

1 fast Jump towards player. 3 projectiles attacks with 0.5 seconds between attack.
2 seconds recover.
repeat.


25%:
Size changes to 200%
Spawns 2 Slime adds (Monster B from Concept for now)
Movespeed 50%

King Shoots 5 projectiles, 0.1 sec delay Boss doesnt aim for the player directly instead shoots randomly in the general direction. runs away for 2.5 seconds. stands still for 1 second and shoot again.
Repeat 5 times then instead of running jump up to 1000 Range away from player and repeat.

0%:
instead of dying Slime king Looses 50% size on every hit taken (1 sec iframe after taking dmg) until reaching 0% size and dies.
Slime King only runs away with 50% move speed.