Lerry the Lebster
All depends on who gets their stun off first
Ashe is reliant on a skillshot for her stun. If she misses with it, she dies. Instantly. That, and later in the game, Veigar can easily just ambush her and two-shot her.
Basically, as Lerry said, whoever stuns the other first wins. But Veigar can use his stun about 7 times more often than Ashe can with hers.
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