Necrophos + Timbersaw

Why this works

Drafting these two cores often forces your other picks into a difficult spot. Both Timbersaw and Necrophos excel at dealing sustained magic damage from the frontline, but they bring very little lockdown. This means enemies can often disengage by simply walking away or using a Force Staff. Without hard stuns from their allies to hold targets down, it's difficult for this pair to secure kills. An opponent can build magic resistance items like a Pipe of Insight and significantly reduce the threat from both of your frontline cores with a single purchase, making it an efficient counter.

This problem becomes most obvious during mid-game teamfights. Timbersaw will dive into the enemy, expecting to be a major threat, but finds his damage gets heavily absorbed. Necrophos tries to clean up, but finds enemies are not getting low enough for Reaper's Scythe to finish the job. The lack of control means your team can't focus a single target effectively. What should be a dominant front-to-back fight turns into a frustrating chase where the enemy dictates the engagement, splits your team, and kites your cores until they run out of resources.