Aphelios is the embodiment of worst ideas for a MOBA. His kit is overloaded, nut not in the classical terms of "engage-shield-dash-disengage" (hello Yasuo), but in terms of various damage utilities. He would fit very well in a roguelike bullethell game, but not in MOBA. Multi-attack with flamethrower? Check. Heimerdinger turret? Check. Ranged multi-target slows with following root from any distance? Check. Dozens of "Devil May Cry"-style shurikens? Check. He is completely unfun to play against even without taking in account his ult. For you as an enemy of Aphelios, his ult is a random between "-50% of your health" and "+50% of Aphelios health".
The only way to counter him currently is to ban him. Oh right, I forgot, I already have Senna banned, because instead of balancing champions Riot releases them overpowered and then in most cases just leaves them be or introduces nerfs so small that they're barely noticeable.