Are a group of abilities added to the stack all on the stack, or each resolves before the next one goes on the stack?
The accepted application of MTR 4.2 is that "I pump Pummeler 5 times" is a proposed shortcut to activate the ability and let it resolve 5 individual times.  So you would not be able to eliminate the creature with grasp of darkness (assuming the other player wants to pay more energy to keep it alive.)  Yes this is different from playing the game by a strict reading of the comprehensive rules without applying the MTR.

The reality is that 5 individual activations is the 'correct' line of play in almost every scenario that would come up in real games.  It's pedantic to make people sit there and say.

"I activate my pummeler once, do you respond?"

"I activate my pummeler once, do you respond?"

"I activate my pummeler once, do you respond?"

"I activate my pummeler once, do you respond?"

"I activate my pummeler once, do you respond?"

In order to perform the obvious line of play people intend in >99% of scenarios.  Around 2005 or so DCI started shifting judging policy away from meticulous enforcement of every comma and toward facilitating people playing the game naturally.  In the early days of tournament magic you were in fact expected to say it 5 times.

I didn't find a canonical reference for this from a wizards netrep, but here is confirmation from two high quality sources known to be written by people who are actually judges:

http://www.cranialinsertion.com/article/818

http://blogs.magicjudges.org/rulestips/2013/04/shortcuts-adding-multiple-objects-to-the-stack/