What it does:
Sets up priority Sunny Day or Rain Dance with Prankster, as well as crippling opposing Leads with either Thunder Wave or Encore (or both if you're brave).
Why is it good:
Thanks to Prankster, Volbeat can set up either Sunny Day or Rain Dance on the first turn and then either cripple the opposing Lead with Encore and Thunder Wave or switch out with Baton Pass or U-Turn. Actually the first two steps should be reversed if you want to cripple the opposition as it will use up an extra turn of your weather. This allows other team members to either set up or sweep while your opponent swaps out their lead or tries to stick around to lay down hazards etc. While it doesn't get a defensive boost from holding an Eviolite, investment in HP, Defense and Sp.Defense with and Impish nature allows it to survive at least one hit while it sets up and retreats.
What counters it:
If the opponent chooses to lead with common Stealth Rock setters such as Regirock or Golem, they can easily threaten or KO Volbeat with Stone Edge or Rock Blast. Not to mention its weakness to Sealth Rock reducing its chances of switching in later on. Being 4x resistant to Fighting helps, but Volbeat is certainly no bulky supporter so you can forget about it sponging Close Combats, should you need to set up your weather again. Powerful Ice, Fire and Electric attacks will undoubtedly KO Volbeat but you should shouldn't be sending it in on any attacks like that anyway. Basically anything with priority or decent offensive presence is enough to threaten Volbeat.
Volbeat @ Heat/Damp Rock
EV's: 248 HP/ 252 Def/ 8 SpD
Impish/ Bold Nature
-Sunny Day/ Rain Dance
-U-Turn/ Baton Pass
Bold nature can be used if using Baton Pass although it wouldn't make much difference. On the other hand, a slow U-Turn allows your weather sweeper to switch in unscathed. Alternatively the Baton Pass set can be combined with the weather setter, which focuses more on boosting your sweepers Sp.Attack rather than crippling your opposition. Sunny Day (Rain Dance)/ Encore/ Tail Glow/ Baton Pass
could be used but I find that Volbeat is trying to do much at once with this set.