You get it by forging in the factory in downtown.
Looks like every other weapon either gives evasion boost or faint boosts. His final weapon boosts evasion by 15%, but most of them boost it by 10%.
Does Insight exist in Crossbell? I'm not sure, but I think that it does. You can use that to augment his avoid until you get him to 100+ evasion.
But going all in is very worth. And very fun. Braindead unga bunga strats are fun, actually.