Giving them a modified form of the heavy mine launchers trigger ability.
You still click and then after a short delay the shell explodes but instead of having a massive AOE to make the aim being to have the shell nearby at the end its simply only armed during the second half of the delay.
Essentially you click to arm the shell, click too early and it explodes in front of them, click too late and it hits them unarmed.
Ideally it would go along with a heavy shift of the damage towards the AOE. Light Flak would probably need to have fewer shots to allow time to arm them.