Your level is the reason you're having so much trouble.
You can continue to struggle as is, but you'll be much better off if you go back through Wizard City and Krokotopia and do all of the side quests you skipped (including all of Colossus Boulevard). This will gain you enough experience for about four levels, plus a lot of gold and items that can be sold for gold.
Why are four levels important?
1) You get more Health and Mana.
2) At level 28, you get Tempest, which is an attack-all-enemies spell and is the go-to for Storm wizards.
3) You'll pass level 25 and be able to get better equipment than you can get at level 20.
Your level is the reason you're having so much trouble.
You can continue to struggle as is, but you'll be much better off if you go back through Wizard City and Krokotopia and do all of the side quests you skipped (including all of Colossus Boulevard). This will gain you enough experience for about four levels, plus a lot of gold and items that can be sold for gold.
Why are four levels important?
1) You get more Health and Mana.
2) At level 28, you get Tempest, which is an attack-all-enemies spell and is the go-to for Storm wizards.
3) You'll pass level 25 and be able to get better equipment than you can get at level 20.
Agreed. At level 25 I was struggling to defeat a 1,000 health boss. By level 27 or 28 I could do 2,000 health bosses with almost no problem.