Ritual Skill is not really useful to level up a solo Asmodeo, to low XP for using it one time a battle. Later on if you can use it 3 times (with 3 Points in it and the skill Summoner at Mage tree) and you can heal yourself (=Asmodeo) with it, it is quite good, but only for the heal, 30 XP for one use at a time that you get 3000 XP with one attack is not good.
But you can make one or two Level (not level of Asmodeo, Level of the skill - it is not one singel Level of Asmodeo that you can make with it if you use it every fight at the first island), but at the beginning you don´t have any undead unit at the first Island and later on it is really not good, a attack is better to level up, because later on you get a chance to attack again after a attack and you can e.g. make 6.000 XP instead of only 30 XP with one Ritual.
Till Level 3 of the hero you should have the same level at asmodeo, after this asmodeo has always a higher level. But only if you use only him for battle and no other unit, because with other units that are beaten (Asmodeo gets XP if a enemy attacks him) or if you attack with other units you rob him many XP.
But if you make all non fighting quests at the beginning and make your hero level 17 bevor you fight anything it can be that Asmodeo is long time lower level
