There's always the Alpine spearmen who're decently well armoured though. And then of course the sword-and-spear elites, but those are hardly "line" troops...

As for skirmishers, isn't that what the Imannae are for ? Note that the Celts, or at least the warrior class, apprently rather disliked the idea of ranged engagements in general by what I know of it (the usual macho thing about proving your viri- er, prowess face to face)...

As horse goes, well, them's the breaks. One recalls though that if the Roman accounts are to be believed the Equites usually seem to have manhandled the Celtic horse in head-on engagements, and even against superior numbers - an outcome unlikely to be attributable to much else than flat out heavier gear and training oriented for shock action on the Romans, as they weren't noted for exceptional skill as horsemen.