Is your playing style close to the table, or do you generally prefer to play back?
Loopers look for any deep ball to attack, so obviously you want to try to keep the ball short. If you are close to the table, pretty much your only option is to block. It takes time and a feel for blocking, as well as fairly quick reflexes. Find a hitting partner to practice with and just start blocking his/her topspin shots to get a feel built up. Start slow and work into getting it faster and spinnier from your practice partner. If you can practice in a drill-like manner, you can focus on the shot itself instead of reflex speed, and work on the reflexes as you advance, perhaps by altering the location of the topspins coming at you on both sides of the table, but initially stick to one side.
If you are back further from the table, you cannot block effectively, so your options are counter-loop/topspin, chop, or lob. If your game is advanced enough to counter topspin, then either aim it directly at the players mid-point, which will make them have to quickly decide to move/adjust, or if possible, obviously aim it where they ain't.
Chopping a topspin, especially a heavy topspin, and really especially a heavy topspin combined with sidespin takes a LOT of practice, but once you get a feel for chopping, it's great to have in your arsenal, and it can be fun, too. You don't see too many defensive players around for the last several decades, so that should tell you how hard it is to master.
Back to the initial strategy: loopers are usually hoping for a "third ball attack" meaning they serve, hope that you push or otherwise return the ball deep, and then they can loop. It's impossible to loop a short return, so I can't over-emphasize the importance of learning to master the push shot.
When I first began playing seriously back in 1975, my mentor the late David Dickson, Jr., told my buddy and I that if we mastered the push shot, everything else would come easier. I didn't fully understand it back then, and let me tell you, push drills are BORING! But... he was absolutely right. As my buddy and I spent hours upon hours just pushing the ball back and forth, we both accelerated our games very rapidly, and it wasn't much longer after that that we were trading forehand and backhand drills at very high speed, and later looping and chopping like our mentors.
I hope some of this has helped.