Broodmother + Crystal Maiden

Why this works

Looking for a big kill combo here misses the point. Broodmother's real bottleneck is mana; she needs it to spread webs and build her army. Crystal Maiden's global mana regeneration lets her do this more freely, especially in the early and mid game. It means Broodmother can stay on the map, maintain pressure, and not have to delay her core items by buying lots of consumables. Crystal Nova's slow is also a decent bonus for her spiderlings to connect with a target.

This allows Broodmother to hit her power spike sooner and begin controlling the map. She can farm more aggressively and choke out the enemy cores. The opponent's response needs to be coordinated aggression. Both Broodmother and Crystal Maiden are fragile and vulnerable to being burst down, especially with area-of-effect spells that clear out the spider army. If the enemy can gank them successfully, they can prevent Broodmother from ever getting the momentum she needs from this small advantage.