We've one upped ourselves. Took us quite a few tries but we nailed it.
Here was the plan:
We determined that the best way to accomplish this was to play "Beat the Clock." The clock being the beholders. We have to get the big dog ready to kill before the beholders make it to the middle. This requires a strategy that outputs the highest DPS in the shortest amount of time.
Tethtoril would run in the middle to get the dogs, go kill the mindflayers to pick up the first three stones. I would hide in the middle and make sure I didn't get seen by any renders or the big dog.
After Teth got the first three stones, he came into the middle and charmed one of the dogs, then cleared the renders before charming the other two. We did this so that the one dog would be the first to pop and we would have plenty of time to recharm it with the fourth stone. There is also an off chance that the first dog charmed will go straight onto the big dog which gets us some earlier DPS.
So now we have all three dogs charmed on the big dog, I immediately sing songs and haste them along with mass bulls scroll. The other important way that we were able to boost the dogs' DPS output was that we both had maxed out halfling heroes companion.
Teth stays in the middle until the first two renders come to the middle and takes them out, then he goes to get the fourth stone. By that time, Teth has a few more renders to kill while I'm spamming heals on the little dogs.
When the first dog pops, Teth recharms it. We can then ride the dogs the rest of the way.
When it is time to strike, Teth gets in there and melee's here while I keep him and the last remaining dog alive.
Had a little fun with a remaining mind flayer at the end.