Hmm, I agree with JMG actually: this is an incredibly fun mana cost, and so it'd be much better for it to be on a great fun card rather than one that's so dedicated to a strategy most players consider "unfun".

It's interesting to contrast with Reaper King, another card with a unique five-colour hybrid cost: why does that feel fun where this feels unfun? Partly that makes you jump through rather more hoops. And partly because "destroy target permanent" feels far more epic than "destroy target land". The former can be used as mana denial, but the latter gives you no choice except to be used that way.

This is pretty close to an "I win" button, but it's at least a 5 cost tri colour; and takes a few turns (or an enormous token drop) to get going. Scary though.

Well, to be fair, I think part of the point is to punish your opponent for playing creatures. This card isn't intended to be fun to play against, but I'm quite sure griefers will like it. It shouldn't be anywhere near common, but I assume theunansweredquestion knows that.

Still, I got to admit, if the ability is supposed to be such a griefer, then it probably shouldn't have such a 'fun' casting cost. I assume a number of people would trip on that fact. Also, what's the weirdest payment you can make for this spell? I suppose. Green has LD, and so does black, but I doubt that green would tie land destruction to creatures. Black might.

Your opponent will never want to play a creature again, because you will always pick which land is destroyed. And then every creature you play, you get to choose again. What if it read,

"Whenever a creature enters the battlefield, that creature's controller destroys target land of their choice."

Yeah, it doesn't even have haste for if you cast it after an Earthquake for 10...

Potentially overcosted; potentially 19/19. I like it. Except, well, it's almost always only useful for blocking.

This is amusing as a one-turn "Graargh, you hit me!" But it's got to die at EOT, so it's pretty overcosted compared with Ball Lightning

Yeah, that's crazy.

The last suggestion reminds me of Death's Shadow.

Make it a */*+1 otherwise, it would die in between players turns.

Or if it's power and toughness is supposed to grow with the more damage that has been dealt:

Change it so that it reads, "Flash. Anger Elemental enters the battlefield with X +1/+1 counters, where X is the total amount of combat damage dealt to a player this turn."

Or even, "Flash. Whenever a player takes combat damage, Anger Elemental gets X +1/+1 counters, where X is the total amount of combat damage dealt to a player this turn."

You could even make it like this: "Flash. Anger Elemental gets -1/-1 for each life you have. 20/20" So the more life you lose the bigger it will get. But if you gain life it will get smaller.