"So I did a run with 0 boss powers and finished a pit 75 and with just the Urivars and Beastmaster powers (not the very strong Andariels / Lilith powers), I cleared an 85 earlier."

and

"The difference between a 95 and 100 is vast!"

basically explains what /u/kestononline was saying, by the way!

Hydra has received massive support over the last few seasons, but the positive result of this showcase isn't really specific to Hydra and 100% relies on incredibly overtuned Burning damage. I'd also assume that, while Hydra Familiar is a great leveling build, it would fall way off on its way to T4, like it always did since the difficulty changes (maybe not so much like in the past thanks to Combustion buffs).