10X better than its actual design
Simple, if you are in the Chaos Covenant, you might get randomly selected to defend the Bed of Chaos. The power of the chaos flame engulfs you and you become the new heart of chaos.
Mechanics are that giant thing will be assisting you in the fight the entire way:
- The arena is redesigned to be divided into 2 sections. The upper section is the normal arena with the giant thing, with the outer edge having a small gap in it. The moveset of the arms is changed to simply be forceful moves that will push you off the edge. The arms have 2 offensive moves. When you're up close, they will attempt to smash you. When you're in the lower section, they will unleash lava waves (somehow) that will deal heavy fire damage (Orange ring cannot protect against).
- The lower section is a lava filled area, the way out is through a breaking the floor above by smacking pillars. The floor above will start collapsing and leaving platforms you can use to climb up.
- Upon the boss reaching 75% HP, a scythe arm (no cutscene) will grow. This arm will have the same moveset as it does now. The scythe arms can slash through the lower section. If the boss regenerates HP over 75%, the scythe arm disappears.
- Same process with the 2nd scythe arm, going in at 25% HP. Firestorm hits though both sections.
- Upon hitting 50% HP, all the unbroken pillars will break (similarly as to how the ground collapses in the game)
Tl;dr
With the boss being a specter-type enemy that is chasing you down (and you must kill), the huge body will attempt to push you over the edge to a lower section where it can unleash powerful attacks on you. Your only escape is finding an exit quickly so you can have better level ground against the specter.
So, feel free to discuss how YOU would have wanted this sad excuse for a boss...
(See what I did? I gave an example so you don't have to post "give us an example")