This will give you a general idea of the heroes you can use when battling in Aspen. Most often the heroes you will use have a self healing ability. This guide is mainly for those in Early Game.
norma with a bit of rnd luck a 5 star norma could get you to Easy 50. A norma 6 Star can pass Easy 50 with help from other heroes. 9 star 32px norma could take you to Easy 100 as long as you have 6 star gear and a priest artifact equip. vesa same as norma above but is more likely to take you to Easy 100 at 6 star and up with minimal help from other heroes.
Other heroes that will help or assist the above two main heroes in get you to Easy 100. Heart Watcher as long as you have crit artifact to assist with her self heal. Lutz will assist with Mage, Dantalian will assist with warriors, dominator can assist with single enemies but is not strong enough to get through walls (3 or more Enemies), Ormus is another healer option but this hero is not strong enough to get through some of the walls like Vesa can. Blood Blade can assist with rangers, Eddga.
Once you have past Easy 100 your 9 star Norma has past their usefulness for Aspen. Vesa will most likely be more helpful with a few other 6 star and higher heroes. 32px valkyrie is useful for walls but you will need to watch her health and use potions regularly as her self heal does not kick in unless she is being controlled (eg DA, Iceblink). AT 9 stars Valkyrie can clear Normal 100 with a bit of rnd luck and assists with other heroes. The 6 to 9 star versions of the heroes Heart Watcher, Dantalian, Ormus, Lutz, Blood Blade are all options as well.
Most often for these levels you will require at least a 10 star hero. Your first choice options would be valkyrie, Sigmund, Karim, Heart Watcher, Vesa, Jahra, Skerei, Barea, Faith Blade, King Barton, Mihm, Gerke.
These will get you to lvl 50 solo. This is assuming 6* gear. If you have a hero that has got to through to Difficulty 50 solo please let me know in the comments and i will add them below. valkyrie, starlight
Special Note: Sigmund Does not have a self heal so you will need to use potions to keep him alive, but he has been know to get up to Hell at 10 stars.
If you know of any other heroes that should be mentioned let me know.