Seriously. An 8/4 for 4 mana is already very good, being in green for easy trample and being outside of lightning bolt range. So a blocking creature and a lightning bolt will do the trick but then the opponent gets another attack phase with all his creatures??
Does this card seem strong enough to see play in many formats? Obviously, the body is ridiculous, and the main question is whether or not it affects things immediately or not. I want this to be good, but without any evasion, this might only be so-so.
In standard I don't know. The rest of the formats, I don't think it is good enough.
No hast, no etb or dies effects. 'Just' a gigant body that can't be chump blocked.
'Whenever' always hint at the fact that it can happen multiple times. Stuff that only trigger once are usually written as 'when'.
The (somewhat) balancing factor is, that it is unlikely, that a creature with 4 toughness can attack and be blocked multiple times without dieing.
Why would an opponent keep creatures back from the first block? It can obviously happen through combat tricks. But I don't see more than one additional attack phase happening very often.