This is mainly for the last phase, because the first two are very well explained in the other solution. But i found a really easy way to defeat the last boss.
Ok, before this. Get your spell at a high level. It isn't required, but if you got it at the max. level you only need to drain Solarius energy twice.
At first take your blues and get rid of the blue slime or whatever the hell it is. Then you get your green minions and take out the 3 pimples at the back. This only takes a few seconds if one or 2 green hang on each pimple. I would also say you put some money to get all the minions on a higher level.
Now he will start puking enemys. Take all the minions except for the greens and put them on a marker right next to the boss so they will kill all the enemies that spawn and the reds will also shot at the last pimple. Get your greens into the fight when he smashes his head on the ground.
when Solarius comes out, just use the spell to drain his energy. As said before, I highly recommend upgrading your magic, because it you will have to do this 4-5 times instead of just 2.
After Solarius is back in the monster, take the greens and go to his back to attack all the pimples that spawned again. Just repeat this. If your fast he can only produce one or two enemies.
I just wrote this, because I got really frustrated with the last phase before dicovering the method.