Eternum Ex (UP)

I found a good strategy is to stay under the bottom platform.

You damage the boss by striking the red balloons that grow from the pods.

Stay on the ground and continue to kill any flowers or balloons on the bottom level or on the first platform, shielding under the platform to prevent getting killed by any flames from the higher flowers.

When the boss was down to one heart no more balloons spawned on the bottom level so I had to jump up and kill a balloon just under the boss.

See clip