This is a bit of a weird one: I want a unit to not run away when attacked by a unit it cannot fight back against, but still attack enemies it CAN hit while being attacked by that unit.

Solution?

Is it able to move normally?

Yes. For all intents and purposes, consider the unit a zergling. When an air unit attacks it, it runs. I don't want it to run.

hero units would work if the attackers are ground units only (like traps).
but heroes try to escape from air units which they cannot attack, or?

The only thing I can think of is flagging the weapon you use for ground attacks to attack air while leaving out an air attack in iscript. This would make it stare retardedly at the air unit.

A good solution, but it doesn't quite cover the scope of what I'm trying to do. Try assuming now that the air unit is invincible

No, because then it will attempt to attack with a non-existing iscript header and it will most likely crash.
You can still do wait 125 and then send them to repeat air attack that does nothing but just wait but the problem is that it will still probably attempt to attack the air unit and won't attack grounds.

Did you try playing with the Infested terran actions? If I recall correctly they go suicide on stuff even when they're being attacked. Maybe when they get attacked by air they run and then come back if attacked by ground?

Refer to the previous post; it was made while you were posting.

Take it from the forgetful noob, they just stare it like a retard.