D/D/D

The usual D/D/D stuff, but with Wonder XYZ to XYZ-summon Kali Yuga in opponents turn and thus not allowing him to activate anything on the field.

Dark Contract with the witch is included in case you already have full combo in hand. If you end up with Siegfried on field and opponent activates Dark Ruler no more, you can chain Dark Contract with the witch and then chain Siegfried to negate Dark Ruler no More.

Standard Combo:

Try to get D/D Lamia, D/D Swirl Slime, D/D Servant Kepler and D/D Necroslime in your hand.

Activate Swirl Slime to fusion summon D/D/D Flame King Genghis
Normal Summon D/D Servant Kepler, search Dark Contract with the witch, if you have all combo pieces. Otherwise search Dark Contract with the Gate and get a missing piece with Gate.
Banish Swirl Slime from GY to special summon D/D Lamia
This activates Genghis' effect to special summon Necroslime from GY

(You should now have Genghis, Kepler, Lamia and Necroslime on field)

Synchro Summon D/D/D Gust King Alexander with Lamia and Genghis
Link Summon Cross-Sheep with Necroslime and Kepler
Activate Necroslime in GY to fusion summon D/D/D High Flame King Genghis (using Necroslime + D/D/D Flame King Genghis) - Important: Summon to cross-sheep Zone!
This triggers Gust King Alexanders effect to special summon Lamia back from GY. Don't use Cross-Sheep yet!
Use Lamia and Gust King Alexander to Synchro Summon Crystal Wing Synchro Dragon. Important: To Cross-Sheep Zone!
Trigger Cross Sheep, to get Lamia back on field. That triggers D/D/D High Flame King Genghis to special summon Alexander back from GY
Use Cross Sheep and Lamia to Link Summon Halqifibrax and special summon another Lamia with its effect.
Use Alexander and Lamia to either special summon Borreload Savage Dragon or Siegfried.
Then use Lamias effect to special itself from GY by sending a dark contract to GY (e.g. Gate)
Use Lamia and High Flame King Genghis to Synchro Summon Virtual World Kyubi - Shenshen.

Thats usually my endboard. When a negate has been used, you can use Halqifibrax' effect to special summon Formula Synchron from Extra Deck and activates its effects to draw a card and Synchro Summon Sattelite Warrior with either Siegfried/Borreload or Crystal Wing. This allows you to destroy up to 3 cards (3 Synchros in GY) and gives him 1k Attack boost per destroyed card.

 

In case you bricked and can't get to all combo pieces:

e.g. Start hand is swirl slime, necro slime and 3 bricks: Use Swirl Slime effect to fusion summon Flame King Genghis. Then Necroslimes effect to fusion summon the second Genghis.

Overlay them for Highwave King Caesar. He allows you to negate 2 effects that special summon monsters. This is not once per turn!

 

Learning this deck takes a while and I still am not able to get 100% of the potential out of it.