There is no pay to win for the grind, its all cosmetic stuff. It really isn't that much different from InJustice 2 where they had a ton of little customization you can play the single player for. For people who only want to play the MP, it could be a bore which is understandable but then again you can just cheese these towers by creating an AI and have it mine for you while you sleep. I will say, anytime you give people the option to pay real money to unlock stuff, the developers put themselves into this situation but there is nothing nefarious going on as far as the game goes.
After playing a bit more, it seems that a lot of Koins can be earned from the dailies, and you also get a lot for doing the tutorials and entering the Krypt for the first time. Koins for individual matches are very low in komparison. I haven't done much towers yet, so I dunno if maybe they pay out better, but it seems that doing the dailies is well worth the time (I had one for just playing a single match as Kitana).
I'm level 12, for the record.
I did spend a good deal of time practicing Cassie Cage combos using a YT vid I found. Getting skilled at this game requires fast inputs oftentimes within just a few frames of time. One of the kombos that I found required a special move cancel from Cassie's back 1 (or whatever the move is where she back-hands the opponent) into the gunner attack (where the opponent gets shot up into the air and pulled toward you with a few more bullets). It took me a long time to figure out why my inputs weren't working, and I thought I was doing it too fast, when in fact I wasn't doing it fast enough.
Also, perfect blocks seem to require ridiculous timing, and I can almost never get them.