I would not recommend Flash as a move to use even in-game, simply because it does not guarantee you avoiding the hit. I've been through gym leaders and E4 where they mass spammed things like Sand-Attack and Double Team on me, but I still managed to hit & KO their Pokemon (and not with moves like Swift or Aura Sphere either).
Even if you used Flash 6 times to minimise your opponent's accuracy, you're wasting time where you could be using a move to KO your opponent's Pokemon instead. Additionally, Flash will only count when the Pokemon you used it on is still on the field. I assume this Pidgeot is Blue's, and from memory he is liable to switch Pokemon before they have fainted. Even if his Pidgeot stays in, after you KO'd it, all those turns using Flash will be wasted.
What I would suggest for your Ampharos is Reflect & Light Screen, while holding Light Clay. You can get the 2 screens TMs in Goldenrod Dept. and Light Clay on Kanto Route 9. With Light Clay, those screens will last 8 turns and will protect your whole team, no matter who switches out.