Slark + Treant Protector

Why this works

Catching a key target in an Overgrowth gives Slark all the time he needs to build a huge stack of Essence Shift. A rooted enemy can't escape Pounce or dodge Dark Pact, which lets Slark secure a pick-off without much of a fight. This is the main way these two heroes can set up a win. On top of that, Treant Protector can cast Living Armor on Slark from across the map. This global protection lets Slark play more aggressively, recover from bad trades, and stay out looking for kills instead of having to retreat to base.

The issue is what happens between those big ultimates. Treant is a slow, defensive hero who wants to play around his team and structures, while Slark is a mobile hunter who plays deep in enemy territory. They often want to be in completely different parts of the map, so they can't always help each other. This means their success often depends on finding a perfect Overgrowth to start a fight, or on Slark winning his part of the map on his own. The outcome really hinges on whether their long cooldowns line up for the most important objectives.