Leveling up Roggenrola sounds like a good idea. The first gym trainer badly wounded three of my team with those damn Emolga, despite all of my mons having the same level as the flying pikachus. I did some training on my Blitzle and gave the Yamask a try, and everyone except Roggenrola are at least level 24 now.
Yamask actually isn't bad. Using Will-o-wisp to burn enemies is a nice tactic, not only lowering enemy Attack but dealing damage every turn and even opening them up to extra damage from Hex. She's also pretty solid defense-wise, having high Def and SpDef. Maybe not the best choice for fighting Elesa but I doubt she'll get permanently boxed.
ETA: HAH! Oh man, that worked out well. I got Roggenrola to level 25, at which point she evolved. I decided to challenge Elesa immediately afterwards.
Round 1: Boldore vs Emolga. Emolga used Volt Switch, dealing probably 75% damage to Boldore and switching to Elesa's other Emolga. Smack Down dropped it to a sliver of health. I took a gamble that Elesa would use the turn to heal, and I used Smack Down again. Elesa spent a Hyper Potion, but Emolga died anyway. One all.
Round 2: Blitzle vs Emolga. The AI must have known that Blitzle had Lightningrod, since all it did was Quick Attack. Two Shock Waves (the second powered up by Charge and then crit) took Emolga down. 2 all.
Round 3: Herdier vs Zebstrika. Nothing elegant here, just Herdier digging for her life while Zebstrika pummeled her with Volt Switch. Two Digs did the job, though Herdier was down to less than 10 HP when it was finished. 3 all, match goes to me