Attacking under tower

Ok so I was pushing a tower and they sent two to me so I was cut off. but I knew one was low health and would arrive first, so I went deeper into their side so that she'd have to fight me solo, even tho it meant I'd fight under tower and only trade 1 for 1.

well what happens is, I put my mouse over the enemy, and I 'e' her and attack her, I am tristana. but trist e's the tower, tho does attacks the enemy successfully once, but to kite away and survive longer I have to move then attack again and this time trist attacks the tower, then attacks a minion, then attacks the tower. because the tower is so tall, that the champion's only clickable target is behind the tower, and it appears you can't even trick attack move click by clicking on the ground near them because I DID recognize that trist had attacked the tower in time and tried to click the ground, and that's when trist attacked a minion and then the tower anyway and still not the champion.

I thought I remembered there was a keystroke I could use to only attack champions, but I couldn't find it in the settings so I guess I remembered wrong. but either that would be helpful in this scenario, or better yet a way to target based off of the unit's base or well basically the ground area that they stand on, so that the tower's height is irrelevant, and like you can't hide a small ADC behind an enemy Tank (this also has happened on occasion, that I couldn't target a carry because their size was smaller than a tanks and they were standing next to each other so the only frame I could actually click was the tank's) (so like, the circle where the champion is standing, which due to unit collision should rarely if ever overlap, that is where with this option someone would click to target something at where it is standing on the ground, and the rest of the frame is ignored) that would be a good way to fix this issue (yeah it's not a bug really but where else am I supposed to bring this up?)