What is the best way to beat the seventh gym leader in Pokemon Platinum? Please tell me the best stradegy I can use with my Party Pokemon!!

My Party Pokemon are: Twiggy, my Torterra, he's at lv. 40, Star my Staraptor, he's at lv. 40, as well, Champ, my Machamp, he's at lv. 36, Dash, my Ponyta, he's at lv. 31, Pachi, my Pachirisu, he's at lv. 25, I haven't raised him in a while, and, Slasher, my Scizor, he's at lv. 22. Twiggy knows the following moves: Solarbeam, Cut, Sunny Day, and, Synthesis, Star knows: Close Combat, Fly, Defog, and, Double Team, Champ knows: Focus Blast, Revenge, Foresight, and, Focus Energy, Dash knows: Flamethrower, Stomp, Flame Wheel, and, Tail Whip, Pachi knows: Shock Wave, Swift, Cut, and, Charm, and, Slasher knows: Metal Claw, False Swipe, Focus Energy, and, Agility. Thank you for your help!!