training grounds doesn't work properly with phyrexian mana bug
Training grounds doesn't work properly with the phyrexian transform cards from MOM (I think). With multiple training grounds, it should allow cheap transform cards like Captive Weird or Khenra Spellspear to transform for just 2 life. However, it always forces you to pay at least 1 generic. The training grounds (or Zirda or Biomancer's Familiar) reduce the generic cost to just the Phyrexian Mana, then you should be able to pay the Phyrexian Mana by paying 2 life, but when you do, it forces you to pay another generic. I think the issue is that Training Grounds can't reduce the cost to less than 1 mana, and the paying of 2 life isn't coded as paying mana (but it is; Phyrexian Mana is mana).
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[Deleted User] commented
You have to decide wether to pay 2 life or mana for each phyrexian mana symbol way before costs are being calculated (at the time you decide modes, additional costs, etc. for the spell) , which is when cost reducers like Training Grounds apply. If you choose to pay life, then the cost reduction of Training Grounds cannot make the spell "free" as it cannot reduce to less than 1 mana.