if you can get some good strong allies, do quests where you can use less of your own units and max ally units. you will level faster if you want to concentrate on 1 card
i.e. quest with 5 allies and 2 of your own - only use one of yours and get the experience awarded for if you used 2
personally, I would concentrate on Lingzi as a good missile unit with slow skill, cheaper to summon. Dhimitrios is ok at the level he is at and doesn't have any skill. You will also find the slow skill a lot more useful for quests