DeathXDORU

I'm gonna dedicate this description to explain the reasoning on some of the "odd" additions and to puntualize what support this deck needs in the (hopefully) near future.

First let's talk about the lv.3s. I believe this is the best arrangement you can get since many people disregard easily the cycling and draw power of black/purple colors because they are so focused on searchers. At first I believed this deck needed a searcher, now honestly I think that while it's true a new lv.3 would make a huge addition in consistency, Dorumon (BT9-058) is nuts as the on play lv.3, complementing the other two really nice, and of course the floodgate for me is almost mandatory in a defensive deck like this one, it can be situational but when it works, you are forcing your opponent to give you a ton of memory or outright freezing them until they can get rid of it. Mind that this will affect your Defense Training (P-107) and The Apostle of Doom Descends! (BT20-097)

Then let's make a quick review on the 4s and 5s. I wanted more consistency so I opted for Tyrannomon as an optional (and very good) searcher that can also work as digivolution, if you prefer it you can opt for two extra DexDorugamon (BT17-065) tho, in the end it might be better. Then, the lv.5s I opted for DexDoruGreymon (BT9-078) simply because I prefer to evolve this guy from the trash with The Apostle of Doom Descends! (BT20-097) as a fix when I can't find DexDoruGreymon (BT17-067) by trashing us one more card and giving one memory, being able to evolve to a level 5 from the trash for 2 which is huge if you think about it, and also delete a cost 4 or less which honestly it's not why we want the level 5 in general. 

Lv.6s and 7s ratio is all about having a way to keep the amazing control and resilience this deck has and also being able to finish games more quickly. I will be testing anyway, but so far I have not found any trouble into finding any of them and they are enough to also not brick your hand by drawing 3 of each. 

The rest is pretty standard, you could run any other of the tamers rather than Tomonori Ryusenji (BT17-090) but I'm just testing him and if you really focus on Dex/DeathX he becomes a very good gameplan accelerator. 

On what support this deck needs is:

1st: Dual tamer with Kosuke and any other. Most urgent of the support. 

2nd: Better Dex lv4s and lv5s specially, they are very outdated, and while the lv4 still holds up, they slow the gameplan and makes them vulnerable.