Alexstrasza is designed to provide a huge amount of raw healing, therefore she struggles against enemey Heroes that have access to healing reduction effects (such as Deckard Cain with Emerald at Level 7).

Another way to pressure a team that has Alexstrasza as their only Healer is to pick a Mage who can provide AoE damage and/or burst damage, especially if at the same time (for example Jaina with Ring of Frost).

Crowd control effects—including Displacement effects (like Face Melt by E.T.C.)—that can connect on multiple Heroes have the potential to heavily punish Alexstrasza's allied Heroes that gather around Abundance while it is loading because they will get nothing out of it other than a big mark on their head.

Heroes with high survivability that can stick to Alexstrasza in the enemy backline and interrupt her attempts to cast Dragonqueen and/or Cleansing Flame can put serious pressure on her.

Abilities that can isolate her to take down the rest of her team are pretty strong as well (for example Cocoon by Anub'arak).