Generic typal cards like Morophon, the Boundless, Etchings of the Chosen, and Metallic Mimic support your Phyrexian creatures
I mean, maybe people start playing graveyard hate, but at the moment, that plan is real hard to beat going long
Seton, Krosan Protector If youre looking to build a druid tribal deck, Seton, Krosan Protector is a pretty good option for a commander
Even though the humble Negate may seem like a better exchange of mana-to-possible-counter, remember that opponents are often tapping out to cast their huge Fireball and not expecting to see a Spell Pierce countering their spell as well as a Counterspell would, anyway
Achilles, on the other hand, rewards you for going Assassin Tribal
While it has some control elements to protect the board, its more of a slow-burn, value-driven deck unsuitable for fast-paced or cEDH games