Guys.... the OP said it

"However, these slingers were veterans, and had accumulated 4-5 chevrons. So I was facing slingers with 6-7 attack + AP"

Remember they also get morale boost with that extra expireince so they WILL NOT rout easely.

Its no suprise he go his Romani manhandeled by the Gauls.

Tip: Don't F* With Anything North Of the Alps