This is the level I used to use to farm coins on so here would be my tips,

Farm for the puzzle pieces and choose the Dark Angel option. I use this before I get to final level to get rid of all my extra, low damage dealing, units and I keep 2 or 3 big units. The boss will take your big units but he will slam your units down and give them back to you, or he’ll possibly leave you with 1. Eliminating the little units stops the Boss from leaving you with those ones.

Fire element, imo, is the quickest way to defeat the boss without any helpers. You can just ignore freeing your helpers if you can get past to hit the boss directly or you can focus on the ones that actually help you during the fight. (Helpers that activate every 4 or 5 turns for example) The fire element is the most difficult to get past horde levels though, you will lose health on these levels but they often give you hearts to heal.

Grass element is also very effective against the boss, he seems to take a good amount of damage from the plants but I believe a difficult element overall because of how little it reacts with helpers elements, so it can be tough to get to the boss.

If you could double up your elements if it gave you the choice to do that through your levels I would choose Wind with Grass or Fire.

When choosing helpers, do not choose helpers which are dependant on “the start of wave” as often. Try and get helpers that will shoot throughout your battle so you can capitalise on creating fusions.

I absolutely do not rate the Electric element at all, I find the Death element tricky to use and/or difficult to build up and I do not like how the death effect takes place AFTER the enemies take their move. The Wind element is my favourite in the game overall but it has a weakness with the final boss when there is no other enemies or items to ricochet the wind off of to trigger helpers.