There's a slight problem in your theory - you will usually not be able to summon the bats in a position more than 6" away from opponent's formation (especially if he played against Druzhag before and knows what he's doing), so your bats will be slowed down to 6" movement (12" AtD with a -1 to courage test, so you're passing just 58% of the time with C3). Next turn calling Heroic move costs you Might which you'd rather be using for more summons too (I know you can call it from Durburz, but still it's a Might point which wouldn't need to be used if you don't send Druzhag there), plus even if you call one, opponent can counter and 2-3 companies of bats die instantly to multiple strength from corruption and generally to any offensive spells, so you're risking a 50/50 to die in an effort to charge someone from behind with S2 attacks which at best will be killing on 5s, usually on 6s against things like Morannons, WoMT, Elves and most of the other commonly used stuff - you might aswell summon a Warg Pack and charge from the front, though that's a bit stupid when you can summon Spiders who will always do greater damage. So in fact your safest bet would be to summon Broodlings because at least they will survive any non-automatic magic effects and they're much better in killing in combat than BS are.
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position yourself to get the perfect tremor off
Any formation in range can use a single might point to resist the spell completely, so again it works 50% of the time.