I'll just list the interactions:
-Pyrohemia + Sprouting Phytohydra
Gives you a Fibonacci increase in Phytohydras.
-Leyline of Vitality + Sprouting Phytohydra
Gain 1 life whenever hydra blocks. Conveniently also negates Pyrohemia hitting you.
-Fungus Sliver + Pyrohemia + Hivestone
Everything gets stronger when you hit it.
-Fungus Sliver + Pyrohemia + Hivestone + Leyline of Vitality + Sprouting Phytohydra
A combination of all of the above. Your field grows massively and your opponent takes as much damage as you have mana. You'll have n+1 phytohydras with p/t going from n/n+3 to 0/3, gain n!-n life and your opponent will take n damage, where n is the amount of mana you pour into Pyrohemia.
-Vent Sentinel + Above combos
Massive damage.
-Apocalypse Hydra + Overgrown Battlement/Axebane Guardian + Above combos
Huge hydra.
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It is not uncommon to wind up with several thousand phytohydras after using Pyrohemia on two separate turns.
If my math is correct...
Ex: Play Hydra. Pyrohemia for 3.
Hydra takes 1. +1 Hydra for total 2.
Hydras take 1. +2 Hydras, 1 Death, total 3.
Hydras take 1. +3, 1 Death, total 5.
Next turn:
Pour all mana into pyrohemia. Pyrohemia for 16.
Hydras take 1. +5 for total 10.
Hydras take 1. +10, -5, total 15.
Hydras take 1. +15, -5, total 25.
Hydras take 1. +25, -10, total 40.
Hydras take 1. +40, -15, total 65.
Hydras take 1. +65, -25, total 105.
Hydras take 1. +105, -40, total 170.
Hydras take 1. +170, -65, total 275.
Hydras take 1. +275, -105, total 445.
Hydras take 1. +445, -170, total 720.
Hydras take 1. +720, -275, total 1270.
Hydras take 1. +1270, -445, total 2095.
Hydras take 1. +2095, -720, total 3470.
Hydras take 1. +3470, -1270, total 5670.
Hydras take 1. +5670, -2095, total 9245.
Hydras take 1. +9245, -3470, total 15020.
That's 30040 toughness in between you and your opponent. That's 15021 potential mana out of a single Overgrown Battlement.
That is a BIG Apocalypse Hydra.
And whatever slams into it only makes it tougher.
This isn't even factoring in Fungal Sliver, which would change all the losses to 0 since they'd grow when they took damage. (Additionally, all your phytohydras would be in tiers of "which came first?" since they'd all have different p/t)
Also not factoring in Leyline of Vitality, where you'd have gained 23632 life assuming it didn't also give a toughness bonus. (Toughness bonus would have delayed the losses by an extra level, so the +10 for total 15 would actually total 20.)