Akali and Karthus have proven themselves as a great Duo Carry setup. Both of them are high impact Tier 4 Carries covering both AD and AP damage. Their abilities also synergize well with each other, since Akali will deal a lot of damage to the enemy corner, which then gets prioritized by a Karthus Ult to deal the final blow. Make sure Akali is always on the opposite side of the enemy carry and you will win fights very quickly.
In early game you usually run a Superfan Opener with either Olaf or Kennen as your Headliner. For the most part your goal is to field a strong board and rush towards Level 8 in Stage 4-2 to get access to Tier 4 Headliner. Thanks to recent buffs to Viego you now have access to three powerful Tier 4 Headliners that can easily help you stabilize.
Once you're stabilized with Level 8 you want to push for Level 9 and cap out your board with Legendaries likes Yorick. You can also keep an eye out for a second Pentakill Emblem to reach 10 Pentakill as a potential win condition. Avoid aiming for 3 Star Tier 4 units unless you are completely uncontested.