Best way to level up Pokemon is to use training accounts you can find here https://forums.pokemon-vortex.com/topic/9-training-accounts-originally-by-apisthebest/
I recommend either catching a fairy, steel, ground, flying and ghost types as these have types that have no effect to them and can be quickly trained to lv100 with the respected training accounts from any level, others you would have to use battle swapping
all other guides for the game can be found here https://forums.pokemon-vortex.com/forum/24-read-game-guides/