Heal/round and Titan for more Health is the first picks you should make with a solo Asmodeo. It depends on luck if you can make this at the first levels, i have started a solo twice and i have twice at the first Level Heal/round, 2nd level Titan and 3rd Level Heal/round to choose.
If you have Heal/round, Titan and Vampirism at a Maxlevel all is easy and boring, because you can stop all Walls, all Spells, you can kill all alone with Asmodeo.
The key is to kill really all with asmodeo, only spells if it is really a lifesafer. If you kill with other units he is not strong enough for the other islands. If you make your play easy if you use a few other units at the start you have later a big problem.
You can say that a Solo Asmodeo has always twice the Level at the beginning as your hero. And later on leveling is much faster, every two fights you gain a level with him (sometimes if the fight is really hard you can make two levels in one fight).
I quit playing with a solo Asmodeo at Level 68, because he is so strong that it make no fun, really boring...The only problem later are really strong undead units (because Vampirism don´t work) and elfen units if they get the right wind or fire buff at a fight.
But my modified game with 400 % stronger enemies are really fun, all above is not working without cheating, but 400% is possible and really really hard