This is the current build I'm running for Hydra, after hashing out some of it on Cockatrice (turns out I had more openings than I thought) and running it through the SPP FNM...

4x Overgrown Tomb
4x Woodland Cemetery
8x Forest
8x Swamp

4x Arbor Elf
4x Corpsejack Menace
2x Dreg Mangler
4x Primordial Hydra
4x Slitherhead

Other Spells:
2x Abrupt Decay
4x Essence Harvest
4x Farseek
4x Grisly Salvage*
4x Ultimate Price

*Currently being filled by 4x Golgari Keyrune in FNM -- that's what the first build had, and since it's not legal to change your deck at a paper FNM...

2x Abrupt Decay
4x Golgari Charm
1x Grafdigger's Cage
2x Ground Seal
2x Mutilate
4x Tragic Slip

Sideboard needs adjustment.

The first big thing about this deck is that it's extremely single-minded. It runs some small disruption to keep itself alive while it sets up, but that's about it. It also doesn't really deal well with disruption either -- this is the main reason for Golgari Charm in sideboard, other than to deal with large groups of small creatures more effectively -- it also can be used to stop removal, since no removal except Wooden Stake in standard currently kills and blocks regen. It's also good against token decks (if those are still A Thing), killing off Intangible Virtue and Lingering Souls tokens.

Anyway, the deck itself plays fairly simply. Corpsejack Menace, Primordial Hydra, then Essence Harvest. The Slitherheads, when they die, help alleviate some of the costs on the Hydra. The Dreg Manglers, though, are way less useful. Farseek and Arbor Elf ramp, because hitting T3 Corpsejack is a worthy boost, and having more mana to give to the Hydra is helpful.

Not much to really say here.