I have done this on an arrowana a long time ago
Use clove oil as follows to immobilize it and then get some good surgical scissors
How to use clove oil: 1) prepare a solution of clove oil with ethanol (94%): 9 parts ethanol + 1 part clove oli. Keep this solution in a dark bottle, preferably also in the dark. Clove oil must be mixed with alcohol before use because it is insoluble in cold (<15°C) water. 2) use this solution to anaesthize your fish. If you do not have earlier experience with clove oil with the specific species, I would advise to start with the concentration of 40 mg/l. In practice this means that you add 2 ml of the solution into 5 l of water. You should be able to immobilize the fish within about 3 minutes.
Here is the thread.