i take it your opponent was using a tzeentch-themed army as by the sounds of it the termies weren't marked... (if they had the MoT, then your opponent was taking tests he didn't need to!)
overall i find tzeentch the hardest army to face with my guard... sure they're arent many of them, but when each basic marine has daemonic essence, that's harsh! (especially rubric termies

)
cheers!