Nasty
I like the idea of the distraction, and the fact that your clr/rog is invisible. Can you tell us more about the target(s)? Numbers, classes, races?
Assuming a standard party of 4 (wiz, ftr, clr, rog)
Here's another general idea, if the party has 2 or more casters:
Give your villain Combat Reflexes, Quickdraw, spiked gauntlets, and at least 14 Dex.
Position him behind the party, next to the wiz and any other caster.
Initiate the ambush.
The two casters will cast spells, drawing attacks of opportunity from your invisible guy. This is good because 1) the weren't aware of you, so they won't be Casting On the Defensive, 2) you'll probably hit and disrupt their spells, 3) you'll do sneak attack damage, 4) you'll still get your regular action.
Bonus points for having cast an Inflict spell and held the charge, so your first AoO does spell damage + sneak attack damage + gauntlet damage. After the first (Inflict) AoO, Quickdraw your scourge and flay the other caster. Then, since it'll be your action, flay once again (and stay right next to the casters, ready to deliver more AoOs).
(note: if the rules say you can't use a spiked gauntlet to deliver touch attack, then just give your cleric heavy armor and make it a regular gauntlet attack--heavy armor comes with gauntlets)
But.. hmm... I guess it'd be more in line with your villain's concept to summon a bunch of poisonous vipers, then try to kill the party's healer with Poison, hit as many as possible with Contagion spells, then run. And watch. And come back when the party is weakened.
