I really like this deck, with the combo in hand, it's obscenely good at eating things and winning fast, but I feel like I too frequently am sitting without combo pieces, literally unable to do anything. I'm going to test with +2 mummy, +2 rewind, -4 don't know as soon as I upgrade the extra 4 cards
It is quite difficult to improve on the reliability of this deck (not necessarily impossible).
Here is a rundown of why the cards are in the deck in their proportions:
The full combo is really mummy + rewind + SoR + nova + (death pend or (2 * nova)). You absolutely need all these cards to get the combo going. So it is really a 5-6 card combo! No wonder it is so unreliable, right?
So, what to change?
I am pretty sure that mark of gravity is correct. Gravity is the only quanta you want a source of throughout the game, and your pendulums are highly likely to be all destroyed the turn after you play them.
I could see taking out the purify. The purify is a gamble that you will lose to poison or SoSac unless you draw it. You may very well lose to SoSac even if you do draw it, so it is definitely a candidate. It depends on the metagame I guess.
I could see taking out a Shard of Focus too. But remember that it serves many purposes. It protects against lobotomizers and eternities, it destroys shields, it cripples decks by taking out non-mark pillars, it heals with black hole (significant amounts vs. rainbow), and it sometimes soaks up CC. It is very good to have at least 1 during almost every game.
I could see taking out 1 SoR, but the power of the deck comes from dumping as many SoR as possible on the pharaoh. Taking out a SoR will increase your expected turn to kill, and obviously will decrease the chances of getting the combo. It is better to have 2 * SoR + pharaoh than 2 * pharaoh + SoR.
Taking out a shard of bravery should make the deck less reliable, not more reliable. It is bad to draw 2 shards of bravery, but if you don't have the combo in hand, shard of bravery can easily get you the combo.
6 novas is almost certainly correct. I think that 10 novas and no pendulums would be better, if that were allowed. Remember that 2 novas can essentially substitute for a pendulum, and you pretty much need novas for SoF.
So the remaining 12 cards are split evenly between pendulums, mummies and rewinds. The hope is to get one of each in your opening hand. If you had to choose 1 to bump up, it would be the rewind because that is the most useful card to have 2 of.
Good luck!
