Getting enough of a grip to use a scaler is going to be a challenge, if you really want to keep the skin on then use a fork to hold it steady while you scale. If you have a large cutting board with a juice groove then push the tines of the fork through the fish into the groove, it will help keep it steady and give you purchase.
In practice this may still be very difficult, so it may be easier to cut the skin off. It's important to use the right technique for safety. A fillet knife is the best tool, you can also use a long, thin and sharp knife. Lay the fish on its side and push the knife upward through the fish, just under the skin, halfway down the side. Slowly cut away from you, angling the knife to stay just under the skin. Once you are done, spin it 180 degrees and cut from the middle to the outside again. Don't cut through the skin as it gives you something to hang on to. Once you have one side done flip it over and repeat the process.