I just want to make sure that Apex Altisaur and Vigor interact the way that I think they do.
Let's say I cast an entwined Tooth and Nail and put both Apex Altisaur and Vigor onto the battlefield.
I choose to have Apex Altisaur fight one of my opponent's creatures. Let's say it is a 5/5. Apex Altisaur will deal 10 damage to the 5/5 and the 5/5 will deal 5 damage to the Apex Altisaur which will be prevented by Vigor and will put five +1/+1 counters on Apex Altisaur making it a 15/15.
With the enrage trigger I choose to have Apex Altisaur fight another one of my opponent's creatures. Let's say it is a 7/7. Apex Altisaur will deal 15 damage to the 7/7 and the 7/7 will deal 7 damage to the Apex Altisaur which will be prevented by Vigor and will put seven +1/+1 counters on Apex Altisaur making it a 22/22.
I can keep doing this until my opponent runs out of creatures that Apex Altisaur can fight?
The thing you're missing here is that as long as you control Vigor, the Altisaur's enrage ability will not trigger, because the damage that would be dealt to the Altisaur is prevented. So only the fight from the Altisaur's first ability, the enter-the-battlefield one, will happen. That one fight does make the Altisaur bigger, but since no damage is dealt to it, it stops there.
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Okay, I thought the damage was done, then prevented so enrage would trigger. I didn't know the damage was replaced like it didn't happen causing enrage not to trigger.
Let's say I cast an entwined Tooth and Nail and put both Apex Altisaur and Vigor onto the battlefield.
I choose to have Apex Altisaur fight one of my opponent's creatures. Let's say it is a 5/5. Apex Altisaur will deal 10 damage to the 5/5 and the 5/5 will deal 5 damage to the Apex Altisaur which will be prevented by Vigor and will put five +1/+1 counters on Apex Altisaur making it a 15/15.
With the enrage trigger I choose to have Apex Altisaur fight another one of my opponent's creatures. Let's say it is a 7/7. Apex Altisaur will deal 15 damage to the 7/7 and the 7/7 will deal 7 damage to the Apex Altisaur which will be prevented by Vigor and will put seven +1/+1 counters on Apex Altisaur making it a 22/22.
I can keep doing this until my opponent runs out of creatures that Apex Altisaur can fight?
RULES OF MAGIC :
http://magic.wizards.com/en/game-info/gameplay/rules-and-formats/rules