I am level 74 and i can't defeat this boss very upset that they had to put a solo boss in main storyline i'm probably stuck forever as i don't even have forest lord (level 58 spell) since i'm so bad alone i have full waterworks gear and i always play in groups i have zero skill alone so i can't ever defeat this.
Below are some tips that can help both with Mika and with getting your Forest Lord spell. I urge you to get your Forest Lord before you attempt Mika and Avalon.
1) Kill that minion ASAP! Do whatever it takes to get rid of him fast. Then heal yourself if you need it.
2) Beware of the Gnomes! spell that Mika casts, as it can stop you from being able to heal yourself. One thing that can help you survive that is having "Cleanse Charm" treasure cards in your side deck.
3) Be sure you have your Life prisms on you. Place the prism that converts Life into Death damage before you place any Life traps.
4) Buy a bunch of Poison treasure cards at the bazaar (or get them from gardening). Put them in your side deck and use Poison after Poison after Poison on him. These cards will boost on Mika, plus keep away his Death shields. Also, the Life dispels from his Gnomes won't stop you from poisoning him.
5) While the poisons are working on him, blade up and use your Centaur spell. Place your Life prism first, then some traps. Use two or three blades at once. The key is to get as much power behind your hits as possible.
6) After you hit, pause to consider your health levels. Heal yourself if needed, especially if you're down below half on health points.
7) Blade up and hit him again if needed. Check your health. Repeat as needed until you're finished.
8) If all else fails, buying a henchman can really help. I recommend the henchmen between levels 50-70 (because they are fairly cheap yet still effective). The level 70 gorillas are best at focusing mostly on healing and the level 70 fire lions are great for hitting. I think they're 75 or 100 crowns. You can earn a small amount of crowns by watching videos or doing the Daily Assignments.
Most importantly, get your Forest Lord spell first. Not having that spell (and your Gnomes!) is what's killing you now.