I never played that map, i'm talking based on Dark Invasion (wich even has a bar that charges the attack and has dmg and miss% based on the charge amount).
Use Channel Ability for the attack. To trigger the damage just use "Unit - Deal damage" or something like that trigger action.
The part i'm not really sure of is the "Pre-selection" part. How to choose which of the units in front of the unit to attack?
I can think two ways to achieve it:
1- Having a "Pre-selectioN" ability that "locks" the target of your attacks.
2- Using "Player - Selection event" to detect when a player selects a unit.
If it's an enemy attach an special effect to it to let your player know that's the locked target. Also add that unit to a "Preselected[Player number of (Triggering Player)]". Automatically trigger the selection of the player hero and, when you press the attack ability, check the distance between the unit and the other one, if it's too high, do nothing, if it's short enough (180) use "Unit - Damage target" action, where the damage source is the Casting Unit, and the target is the "Preselected[Player Number(Triggering Player)]" unit.
Now... How to deal the damage "naturally", the same shown in the Unit stats... i'm not sure. I guess you would have to keep track of all those values with triggers, or have an invisible dummy with same items and stats of your unit, attack the preselected unit.
3- You could have the target acquistiion range of your hero to 0. Also order it to stop if ti acquires a target. Also, if a unit is ordered to attack-move, re-order to move to the same point without attacking. If you order to attack an specific unit, then enable the attack...