I got duo using a lvl 63 Confessor cleric and a level 15 Sorceror Mage, both with Legendary weopons (Dawnslice and Spirit Leech Scepter / Terreract). Mage had a ring of Wisdom (xp). Average level of monsters was 41. By the time the campaign ended my Cleric had not leveled at all and Mage was level 35.
Was tough at first until the mage got some Intellect for magic damage. Used mostly targetted kills to take out enemy magic users / damage dealers / healers first – mage using lightning or lightning storm, and cleric using smite (to hit back rows and always hits). Did not find the Shadow effective when there were more than 4 enemies (took too many hits). The spirit leech kept the mage topped up with health and power, and the cleric took most of the hits.