Soloing is only completely do-able by certain classes.
The amount of damage a mob does past 20 is far too great for a class such as a mage to solo. Skills aren't kitable for mage because of the casting time and global cool down. Pots have too long of a cool down (30 seconds, you can stack another type of potion that regens health every few seconds but I believe those are only obtained from quests and being crafted.), and player defense is far too little.
You shouldn't have to be upgrading armor to +6 and up at level 20 simply to solo. (And even then, it's still not that easy.)
If you're not a cleric, soloing will take much, much longer. You have to sit to regen health (or drink tea to regen health faster) every 2-3 mobs.
I can't say much for the other classes since I never leveled any of those past 20, but I've heard pretty much every class besides training clerics having at least at least one or two issues soloing.
All in all, this game seems to aim more-so for parties. Even duos make an extraordinary difference. If you want to efficiently solo, go training cleric.