I've fought quite a few Solduros and naked dudes. The naked dudes never rout, but I just kill them with javelins and slingers. Solduros like to kill my FMs, but they usually run when the rest of the army starts going.

But really, usually I face them with one or two FMs, a bunch of levy and regular germanic spearmen, and some germanic archers or celtic slingers and maybe some celtic swordsmen mercenaries. Are my spearmen really superior to regular celtic spearmen?