My opponent has several slaking, and its power keep wrecking my team! how can I beat it without getting destroyed?!
Use slaking's ability against him. His ability, Truant, only allow him to attack every other turn. So use this to your advantage since he has mostly slaking in his party and teach your pokemon protect. Then use it every other turn and fight back then. Since you use it every other turn, it shouldn't fail. You can also use stat raising moves to boost your defenses.
Fighting moves wil win through here. Please write in your team so I can help more with their movesets.