Fletcher once you get how to use him is the funniest merc and a very usefull one. And one thing that you should do is make a bait, so maybe shoot someone, let them follow you and then prevent when they are coming and throw a sticky bomb, this will give you the advantage, also, as how CCP115 said, just jump, use loadouts with lock on or quick draw, I recommend you the A31, or the BL33 or BL32, any on those, Im using the loadout with tough, flying pig and quick draw, for me is the best.
use your EMPIRE 9 in close quarters and the BLISHLOK in long range combat.
Fletcher has a Hit n´ Run playstyle, use corners to stick someone, and then follow them with the empire 9.
Try to prevent 2 vs 1, is possible but is usually hard, because sticky boms aren´t like a grenade launcher, with a huge AoE, so if 2 enemies are following you, use corners to your advantage to stick them.
For spectating people using fletcher in free rotation, something inside me died, one thing that you should do is as soon as you throw the sticky, press E, in close quarters is a viable strat, if you are trying to stick someone at medium to long range, perfom a long jump, and throw a sticky, and wait until you hear a stick sound, then E, but in close quarters usually the bombs will reach the target before it explode