Gardenia

General Info - Gardenia's focus is on the Grass type, and with that type come all the weaknesses it entails. Namely, Fire and Flying type moves (You probably don't have any ice types yet!). What you should watch out for though, as with all grass types, it status conditions. So some antidotes or paralyze heals are nothing but useful in this battle.

Lv19 (Grass) - Cherubi is a fairly week pokemon, and should not take much trouble to go down. If your starting Pokemon is something like Staravia, you should be able to wipe this thing out, no sweat.

Lv19 (Grass) - Turtwig is a sturdier Pokemon compared to it's commrades, but any hard hitting, super-effective attack (like Flame Wheel or Wing Attack) should take it out.

Lv 22 (Grass/Poison) - Roserade is the first fully evolved pokemon you will probably meet in Diamond and Pearl, and, if not handled correctly, can cripple your team. Quickly damage it with a super-effective attack right off the bat, and if your pokemon gets knocked out in that round bring in another Pokemon (if you have one) to dish out a second super-effective attack to finish her off.