I play now at medium level... And want to have some clue about usefulness of AOE attacks at higher level, because all my strategies based only at those type of attack conducted from medium line. Can you help me, please?

AoE attacks are useful to damage surrounding heroes(same line and the heroes behind it) ,depend on how big the AoE was.
Nilya's Ice snare for example, are small AoE attack; and Tareth's Arrowstorm are big AoE attack.

Somehow you need some Line Attack (Like Sylphi's Flapping Wings and Sumiko's Whirlwind) if you want to take down Back Heroes faster.

If you planning to make mid heroes as core killer heroes, better make sure to have 2 tanks as protection to mid heroes (unless if the enemy also use AoE attack, that will be little trouble)

Thanks! Its useful... I not so often use 2 tanks at one time maybe it will be better to try...