Too strong within its own element when you consider stone skin and PA.
Even if it wasn't

PA would still be a viable addition with this and in a stall deck, even without the aid of titanium shields. (especially upped)
A lot of decks struggle to wrack up to barely 25 damage per turn without getting CC'd and over a period of time in the match,
and the upgraded absorbs 75? on top of that you can add even more points equal to half of the unupped+ at the cost of a measly couple of

?
Compare this to Bone wall which has a similar element but in counters, although situational, an Adrenal Frog can take down bone wall in 2 attacks if it wasn't stacked enough.
This can take way more turns potentially despite it being numeric rather than counter based.
and by stackable I assume you can play this again and add the actual value of the card? (75+75?)
A tad too strong I think. I would suggest significantly lowering the points and the cost in order to accommodate the cost of using titanium/diamond shields to add points.
The idea and concept in itself is really cool though, I would like to see a balanced version in game.