It's a little pointless to do comparisons since the duel happened in the middle of the battle and we have no idea in what situation each character was. Cenarius could already been fatiqued or low on mana as he was battling for a long time. He could also have underestimated Grom since he was winning the battle before the Orcs drank the Demon Blood. In a normal situation one vs one without armies and each character knows of the other I believe Cenarius has way more power and magic is a faktor that Grom cannot overcome.