Boss Suggestions

Nether OverLord:
Spawns: Nether
Special Attack:
Flaming Ball of Destruction: Spawns 3 FireBalls that do critical damage and Fatal Terrain damage. On impact the Fireballs will reveal a Hellish HellWorn within the Fireballs.
Burning Body: Fire effects will spread around the body showing that it's healing up and getting near will knockback you off and destroy the terrain. 2 spinning Fire Orbs will spawn around the body and use a bow to destroy it to stop the healing process.
Flame Lines of Burn: A shaky X pattern made out of fire will spread around the Boss.
Strength: If it attacks you. You will burn and take big damages. If it's near fire, it will absorb the fire and extinguish the fire but it will heal up and stregthen itself.
Weakness: Empty Splash potion have water inside so use it on the OverLord to damage and weaken it.

Slimy Muncher (BIG Slime)
Spawns: OverWird
Special Attack:
Split: It will summon medium slimes that are dangerous but the shinken itself.
Ground Smash: Jumps up really high and smash the ground causing Critical Terrain damage. Bunch of Poison Slimeblocks will jump out from the slimy crater and break into slimeballs. (This attack will shrinken itself)
Spit of Slimes: It will spit out a big slime and cause the land nearby to become poisonous slimeblocks which will later shrink to a slimeball (this attack also shrink itself)
Additional notes: The more bigger it is, the more stronger it gets. The more smaller it is, the more weaker it gets.
Stregth: If it touch slimeballs, it will become bigger. Touching it will give you damage and inflict slowness to you.
Weakness: Everytime it attacks, it will shrinken. Make sure to keep the slimeballs as it will grow if touching them.