Slime Boss

Slime Boss enters the battlefield with a number
of +1/+1 counters on it equal to the amount of
mana spent to cast it.
Slime Boss isn't legendary if it's a token.
If Slime Boss would take damage, instead remove
that many +1/+1 counters and create a token
copy of Slime Boss with a number of +1/+1
counters on it equal to how many +1/+1 counters
Slime Boss has left.