Here's a similar deck I've been working on:
6u5 6u5 6u5 6u5 744 744 744 744 744 744 744 744 744 744 745 745 745 745 74b 74b 74f 74f 74f 74f 74f 74f 74o 75m 75m 7n2 7n2
I only have one gravity nymph, but more would help the deck. The deck is playable without the nymph, either run it with just 30 cards (what is shown, without the nymph), or you could add another charger. I've posted it fully upgraded, However I don't have all the towers upped right now, so I can't give a true opinion on how it will work. What I have right now is:
6u5 6u5 6u5 6u5 55k 55k 55k 55k 55k 55k 744 744 744 744 745 745 745 745 74b 74b 74f 74f 74f 74f 74f 74f 74o 75m 75m 7n2 7n2
Anyways, I've found the shield to be unnecessary, seeing as your quantum denial will almost certainly stop your foe from playing anything out of the oty's range. The two pendulums help to get discords out faster, but they could just as easily be replaced with two more towers.
The main problem with this deck, and with any black hole mono/duo/trio is that gravity is so damn expensive. Every card costs a ton, making these decks slow. Even with all upped towers, it would still have speed problems.