Enemy Ranged -> Enemy Heavy -> Friendly Heavy
He targets the first Enemy (Ranged), essentially killing all the remaining minions including his remaining friendly minion.
What will happen is he kills the enemy Ranged and the friendly Heavy.
Enemy heavy Survives and the lane is pushed towards the spellsword's throne.
You can never kill both the last regular minion and the heavy from the same team with the same attack since it's not resolved "one-by-one". All minions are hit at the same time. And when that happens, the heavy minion is not "the last remaining" and so is not harmed by this attack.
What happens if all minions die at once?
The only way for this to happen is if the only minions remaining in the lane are the two heavies and are both hit with the same attack.
This is technically possible, but extremely unlikely.
If that does happen, then, naturally, the lane is "reset".
If the tower variant is used, both teams will score the Tower token.