Making a Boss Mob

I would like to make a boss mob which has 3 phases. It is called the Red Golem

1st phase

• It does generic melee attacks.
• Once in a while, he will summon a flying meatshield that will follow the player. When there is a meatshield, the Golem is invulnerable.

2nd phase

• It summons soldiers called stone minions and does not summon the shield anymore.
• It continues to do melee attacks.

3rd phase

• It deals melee attacks
• Sometimes, it will lightning-attack the player
• It will summon more minions
• Sometimes, he will create shielded warriors to deal with you.

It reaches a phase once it reaches certain HP