I've 1v3'd before. It was about 10 minutes in and i was like 1 level ahead of opponents and both my teamates disconnect ( they were going off the same router/connection) and I managed to ace the oppenents a few times with Tristana. it was pretty insane. about 10 minutes later they come back thankfully and we win thegame. i thought it was pretty clutch. Another time it was me as Kassadin and my teamates shaco and Mordekaizer and shaco left like a minute in and still we won the game. (I'm level 21 at the moment and Mordekaizer was around my level too).
I seem to demolish opponents my level when I'm using my strongest champs ( Tristana, Morgana, Malzahar, Kassadin, Ashe, Xerath, I'm a Mage kind of guy, I only go AD Tristana, no other champ.)
But its alot more difficult when against lv 30's that have alot of runes and masteries.