Flash should beat this guy pretty handily. Achillias has some super speed and has grace but I really don't think it's enough to counter's Flash's level of speed.
Well if it's close to WW's he should manage, especially with the grace. I'd think especially in a first fight where he doesn't know all of Achilles' abilities, Flash could lose.
Though of course even if he manages to get past Flash, then there's Diana to stop him.