Lone Druid + Viper

Why this works

Facing down a tower push from this pair can feel impossible. Viper can place Nethertoxin on the objective for constant damage while also breaking the passives of any heroes trying to defend. This pressure is amplified by Lone Druid and his Spirit Bear providing a huge amount of physical right-click damage. It becomes very difficult for the defending team to find a good initiation, because Viper's constant slows make it easy for the bear to chase down anyone who steps out of position. A single target caught by Viper Strike is almost certainly dead.

The problem is that both heroes often want to play the same game. They both excel at winning their lane and then using that lead to pressure objectives, but this means they can end up competing for farm and space. Their combined strength is taking down single targets and buildings, but they don't offer much in the way of teamfight-winning stuns or area-of-effect control. Against lineups that can clear waves from a safe distance or have multiple ways to kite the Spirit Bear, this duo's game plan can fall apart if they don't get a significant early lead.