Generally, anyone under 300k power is very new. 1.5-1.7million is a fairly active player. 2-3 million is active or limited by the cards they have access to getting. I'm about 20,000 off from being in the top 100 cardscores at almost 3.9million, and the top players are about 4.2million.
Your attack power for this event is your cardscore x any multiplier cards you have. A single hit is about 1/6 of that, but ranges widely, and a big hit is that exact number. Now think about the difference between doubling 1/6th of your cardscore, and doubling 1/6th of mine.

Not to mention you get double ep for either taking the biggest chunk out of an enemy, or landing the final blow. Or... triple ep for both. If anyone goes for the multiplier cards for a high rank, they're probably going to get all of them for a 7x cardscore multiplier too.
You'll see in a few days the lightning bosses which will also take the top end limits off of anyone one-shotting a lot of these current bosses with a single stam(boob) point. Not only is their timer only 30 minutes, they also bring the ultimate boss which has 100 million health. With a full 7x multiplier, I can easily kill it with my 6point stam hit 4 times. (More likely 3 and a few one-point hits.) So yeah, like Gromelon said, it's not a very noob-friendly event.