Doing a hydra in a set without +1/+1 counters is pretty hard.

My friend said this card would break legacy.

It's considerably strong in Standard too, considering that Battle for Zendikar is on tap. Swinging with an 11/11 on round 3 sounds pretty good.

By the way, is that +X/+X supposed to be permanent, or until end of turn. Because I would think permanent would be "and gets +X/+X, where X is equal to all cards exiled by ~."

No, the +X/+X was meant to be until end of turn. I imagine my sets to exist in their own separate continuity, so the most expensive card this could hit is at 8 mana, but that's still a 9/9 on turn three and they're milling 8.

I think that this creature 1/1 body and lack of defensive abilities means that just about anything can kill this creature before it swings. Additionally this doesn't have trample so it can be chumped for days (though the mill may become a problem).

Hmm. Well, at the very least, it's easy to imagine playing one of these on round 2 and 3, and consistently swinging with a 4/4 and 5/5 every turn. That puts it at approximately Tarmogoyf level of good. It's not that players can't defend against that... but a good player should be able to supply consistent pressure, making a card like this lethal.

Goyf level is admittedly pretty strong, though this might be slightly weaker as you might hit three lands or low cmc cards (unless you manipulate your library around this card), which seems less consistent than Tarmogoyf. This is also two color, but then again if you did want to use this in legacy you'd have Brainstorm, which would set this card up pretty well. Specifically my friend said BUG would shoot up in power with this card. Jund or any other deck can still kill this with Lightning Bolt, which I think is pretty notable to its vulnerability as it lacks haste. Being a 1/1 the turn it drops is an acceptable drawback, right? If I made a two-colored card with the same CMC that's as powerful as Tarmogoyf I guess that's not a problem since I haven't heard that Tarmogoyf is so powerful that it needs to banned (though I'm sure some have said it, I just haven't encountered such sentiments myself).

I don't know. It's admittedly tricky, and to add to that I'm constantly overvaluing cards because I've played the game too long and sometimes I don't recognize the power creep for what it is.

But personally speaking, I normally don't factor drawbacks that highly when evaluating card power. So the fact that this card is terrible on the block, can be shot down by a Shock, requires two colors and can occasionally misfire are all noted, but I count them all as corner cases. I just see a card that 90% of the time will be a phenomenal beat stick. Something that makes Ashenmoor Gouger look weak and inefficient by comparison.

If you don't see the card that way, that's cool. I could easily be wrong. But the card might be better placed in Mythic. That seems like the best place to stick impressive efficient bombs that could fizzle out... or at least that's how I'd like to see Mythic.

I'm of a different opinion when it comes to the division between rare and mythic. If I see a mythic (as I really only draft) I want that card to easily be something amazing, not something that might be. That isn't to say the card need work on its own with no help as Monastery Mentor is fantastic. The mythic in this set Hazy Phantasms, while being significantly harder to play, feels much more like a mythic to me. I guess it's worth noting that I've never felt Tarmogoyf was a mythic card and that it's rarity in the Modern Masters series was chosen to appease the secondary market.