In some/all games:
Roto Encounter: "increases the chance of encountering high-level wild Pokemon for a certain period of time."
Arena Trap & Illuminate: Increase the chance of encountering high-level Pokemon to 200%
No Guard: same as above but only to 150%.
Also you might want to try to use Honey. Works with this ability.
It says for Swarm: "Increases the chance of encountering wild Pokemon."
Also, Helix716, Magnet Pull and Static don't increase the chance, but just make electric/steel type Pokemon more common in areas where they are.
Hope I helped!