If you're just playing through a game, using Cotton Spore will decrease the opponent's speed greatly, and you can further this effect by using Carbos vitamins on your Ampharos, which boost its speed by 10 EV values. Ampharos also has the static ability, which may paralyze the opponent if it makes direct contact. Paralysis also cuts down the opponents speed by a lot, and Thunder Wave can be useful to paralyze the opponents that don't attack physically. Hope this helps!
