Sorry if this has been answered already, I couldn't find it. What units can hit them? Can another High Altitude unit hit another? I want to spend my event points and I have nothing that can hit the high altitude units, so I'm wondering if the ACE can hit the harbinger unit so I can actually hit those bases.
Seraph, artemis and buzzsaw are such units and can be trained and controlled directly but they are somewhat outdated and can be ineffective.
Some drones are also dedicated AA unit that can hit harbinger such as hoplite drones and dervish drone. Hoplite drones are spawn only by level 10+ hoplites and dervish drone requires gold tech. These two deal in-meta damage which is considerably more than previous 3.
Two special cases are Ares and Matt, both are able to hit high altitude while being able to target ground units. Both deal rather high damage.
Dervish with gold tech is the best option in my opinion because it's aerial which harbinger cannot target directly while the ground based units risk retaliation from harbinger.