this is what i learned from tournament style players as far as their strategy goes, thus, my advice for anybody willing to step it up:
-Timing is key -- do not rush any attacks by pressing any buttons....know your character you choose very well to stay confident and on top of your game.
-Blocking is the most important asset -- I noticed in this game, you can avoid x-ray moves by simply timing when to block them. Also, know your enemies' highs, mids, and lows and therefore you will have a much easier time dodging their attacks if anticipated correctly.
-Combos -- Yes, i agree, simple combos are the best and if you know how to do the simple three hit combos then you are on the right track. Furthermore, if you know how to utilize those combos with a special move follow up then this strategy will also enhance your arsenal. The more complicated kinds are the juggle combos where you have to utilize timing as your main key, these types of combos in my opinion are the most satisfying type of gameplay of all in mortal kombat. With complex juggle/pop up/wall/special combos, you have proven yourself ready for ranked matches online.