I suggest this moveset:
Dragon Tail (since you don't want to replace this)
I suggest SolarBeam instead of Frenzy Plant or Leaf Storm. Although both Frenzy Plant and Leaf Storm both have at least 20 more power than SolarBeam, Frenzy Plant leaves you vulnerable for an attack during your recovery, and Leaf Storm lowers your Sp. Attack by two levels, which makes it difficult for this to do any damage after your third Leaf Storm. I recommend Sunny Day to eliminate the SolarBeam recharge, but you could also use Growth or Calm Mind with this.
If you are insistent on keeping either Frenzy Plant or Leaf Storm, then I recommend keeping Leaf Storm, though I also recommend that you teach your Pokémon another move to compensate for the Special Attack drop. (Although this is the only Special Attack move that your Pokémon knows, I've seen the damage amount drop drastically from this. It can ruin the battle). Here's what I recommend:
Growth or Calm Mind
That way, you can use Growth or Calm Mind a couple times in the beginning of the battle to raise your attacks (or, for Calm Mind, attack and defense) and make Leaf Storm hit with more power before draining.