Just pointing this out:
Meganium (252 SpA/252 Spe) Timid LO Leaf Storm vs standard Tank Regirock (from UU)= 89%-105.5% clean OHKO with SR up.
the same Meganium Leaf Storm vs Special Tank Slowking= 78.1%-92.2% not ideal, but it's still a good OHKO chance with SR and Spikes. At worst, it's a 2HKO
She can run a special set with Hidden Power (Fire) and Synthesis and still KO what she needs to. The SD set is clearly her best option, but this works well under sunlight (2HKO's most Vileplume and has double Synthesis recovery), which is necessary to have in order to get rid of the Sandstorm that inevitably will come with Regirock.
That's just an idea though. I'm not really an expert in this.
Hey folks, long time lurker, first time poster. :)
As an alternative to this, what about Bellossom? It's slightly less bulky than Meganium (75/85/100 vs. 80/100/100) and is quite a bit slower (50 vs. 80) but it's also slightly stronger and has access to Sludge Bomb and Sleep Powder to close the gap somewhat. (Unlike Vileplume, it does learn Leaf Storm.) The purposed Sunny Day set would also work for Bellossom, as it has Chlorophyll to keep up with Vileplume.
Since Bellossom is so slow, I went ahead and used Modest. One Speed EV is enough to outspeed Vileplume and Regirock without any speed investment.
Modest LO Leaf Storm vs. 252 HP/220 SpD Careful Regirock: 102.75% - 121.98%
Modest LO Leaf Storm vs. 252 HP/200 SpD Calm Slowking: 90.36% - 107.11%
Modest LO Sludge Bomb vs. 252 HP/252 SpD Calm Vileplume: 35.88% - 42.37%
There's also Victreebel, but it might be a bit too frail for this.