Pokemon Diamond Discussion
A strong dark pokemon could easily take out Deoxy's, Celebi, Mew, and Mewtwo. From there you would only need an electric or grass pokemon to take out Blastoise and Suicune.
But they are strong pokemon, so they're not completely lost.
If you post your move sets for each pokemon then we can better judge how good they are.
infernape lv 100 flamethrower, mach punch, close combat, rock climb
Luxray lv.100 thunder,thunderbolt, crunch, discharge
Roserade lv.100 petal dance, giga drain, poison jab, sludge bomb
Gastrodon lv.100 surf, waerfall, earthquake, muddy water
Abomasnow lv.100 blizzard, wood hammer, ice beam, giga drain
staraptor lv.100 fly, brave bird, airial ace, close combat
and that's my team they're really good if you know how to use them
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