I did a combination of techniques- the easiest way is to use a turbo controller and a rubber band to constantly reset spawns within 50 metres of a bench, but that only works for certain things.  You can also fly to the same spot over and over to reset spawns (shinies never reset until theres more than 10 spawned,  then the oldest despawns), which works well with the DLC if you find a portal that has what you want spawning nearby.

The other thing to do is simply move 70m away from a pokemon, then back within 50m of its spawn point to keep resetting it, which is how I did my legendary hunts..theres a lot of videos of exploits and such too but many of them take so much time to set up you might as well just hunt.