Half? What game are you playing? There is no champion in the game that can two shot any minion with their base damage. The best any champion with base damage can do is 5-7 on a melee minion, and with a Doran's blade it maybe drops it to 5-6.
I am trying to say that the reason a person trying to last hit should fail, is because they are being bullied away from the minions by their opponent. The measure for a person's ability should be their positioning, and coordination, not memorizing the amount of pixels it takes to drop the minion's health to zero.
And as a good reminder to the farmer, executes are starting to get visual ques, which would be perfect for new players. When the execute would be proc-able, the minion could have a target on them, letting new players know exactly when they should be last hitting the minions. I mean, people used to memorize exactly how much damage Cho'Gath did with his ult, so they could always land it, driving a divide between the hardcore Cho'Gath players and the casual ones, and now he has a big glowing symbol that appears, letting everyone know he can kill them.