When using Zoom to reset you need to select a different Echelon rank or layer. Warping in same map doesn't reset spawn for these random chance mobs. Also getting caught in a battle with other monsters may cause them to despawn. Fall season may be the best since rock formations rise to allow access to some locations. Particularly the Tufty Tot and Liquid Metal Slime on the right ledge.

There may be a few spawn points I've missed. These are the ones I cycle through when power leveling and found while playing. Liquid Metal Slime seems to be a bit rare but there are more points where they spawn. Only know of the 3 locations for Metal King Slime. They can appear in other Upper Eschelon areas as well, but these are the ones I find are easier to hunt for. Also the spawn rate may vary depending on whether you have beaten the game or not. Higher chance if you've completed the game.

Note that spawns are not guaranteed. Zooming to another location and back will reset chances of spawns.