Lion + Slardar

Why this works

Thinking this combo works because of the damage is a common trap. Lion's burst is almost entirely magical, while Slardar's Corrosive Haze only reduces armor for physical attacks. The real value is the sheer amount of lockdown you can layer on a single hero. Slardar blinks in with a Slithereen Crush, and Lion can easily follow up with his own stun and Hex before the target can even react. This gives your entire team a huge window to focus someone down. The problem is that Lion himself doesn't get much direct benefit from the armor reduction, making the damage part of the equation dependent on your other teammates.

This duo lives and dies by finding pickoffs. They excel at roaming the map together, catching an enemy out of position, and deleting them before help can arrive. If they get a lead this way, they can create a lot of space. The plan falls apart, however, when the enemy team starts grouping up as five. Slardar and Lion can still usually kill their initial target, but their single-target focus can leave them vulnerable to a teamfight turning on them once their stuns are on cooldown. It often becomes a trade, which isn't always what you want.