I really don't want to be the bearer of bad news, but it looks like Power Whip + Weather Ball is illegal, since Venusaur nor Tangrowth can learn Weather Ball.
However, on the bright side, Growth appears to be a +2 boost to both stats in the sun. So all hope isnt lost.
Victreebel @ Life Orb
Chlorophyl
Naive/Hasty Nature
252 Atk/40 Sp. Atk/216 Speed
~Growth
~Leaf Blade/Power Whip
~Weather Ball/HP Fire
~HP Ground/Sludge Bomb
The speed puts Victreebel at 251 (502 in sun), to outpace ScarfChomp. A few more EVs into speed for Scarf Kojondo would also be fairly reasonable. A +Atk nature can be used for more power, but you would get outpaced by way too many scarfers (Garchomp, Sazandora, Genosekuto, etc,)
Some Calcs:
+2 Leaf Blade Vs. 252/252+ Blissey : (83.61% - 98.46%)
+2 Leaf Blade Vs. 188/104 Snorlax : (80.72% - 95.03%)
+2 Leaf Blade Vs. 0/0 Garchomp: (95.81% - 113.13%)
+0 Weather Ball Vs. 252/0 Metagross (86.81% - 102.75%)
+0 Weather Ball Vs 252/0 Celebi (71.78% - 84.65%)
+0 Weather Ball Vs 252/252+ Skarm (77.84% - 92.22%)
+0 Weather Ball Vs 252/0 Skarm (116.77% - 137.72%)
+0 Weather Ball Vs 0/0 Jirachi (85.04% - 100.29%)
+2 Sludge Bomb Vs 0/0 Sanzadora (88.89% - 104.63%)
+2 Sludge Bomb Vs 0/0 Dragonite (81.11% - 95.67%)
+2 Sludge Bomb Vs 252/252+ DNite (66.08% - 77.97%)
+2 Sludge Bomb Vs 252/0 Emboar (93.63% - 110.38%)
However, on the bright side, Growth appears to be a +2 boost to both stats in the sun. So all hope isnt lost.
Victreebel @ Life Orb
Chlorophyl
Naive/Hasty Nature
252 Atk/40 Sp. Atk/216 Speed
~Growth
~Leaf Blade/Power Whip
~Weather Ball/HP Fire
~HP Ground/Sludge Bomb
The speed puts Victreebel at 251 (502 in sun), to outpace ScarfChomp. A few more EVs into speed for Scarf Kojondo would also be fairly reasonable. A +Atk nature can be used for more power, but you would get outpaced by way too many scarfers (Garchomp, Sazandora, Genosekuto, etc,)
Some Calcs:
+2 Leaf Blade Vs. 252/252+ Blissey : (83.61% - 98.46%)
+2 Leaf Blade Vs. 188/104 Snorlax : (80.72% - 95.03%)
+2 Leaf Blade Vs. 0/0 Garchomp: (95.81% - 113.13%)
+0 Weather Ball Vs. 252/0 Metagross (86.81% - 102.75%)
+0 Weather Ball Vs 252/0 Celebi (71.78% - 84.65%)
+0 Weather Ball Vs 252/252+ Skarm (77.84% - 92.22%)
+0 Weather Ball Vs 252/0 Skarm (116.77% - 137.72%)
+0 Weather Ball Vs 0/0 Jirachi (85.04% - 100.29%)
+2 Sludge Bomb Vs 0/0 Sanzadora (88.89% - 104.63%)
+2 Sludge Bomb Vs 0/0 Dragonite (81.11% - 95.67%)
+2 Sludge Bomb Vs 252/252+ DNite (66.08% - 77.97%)
+2 Sludge Bomb Vs 252/0 Emboar (93.63% - 110.38%)