Prismatic Gelatinous Cube on Hard tips

[Ender]

This is the second cube you fight. Any tips on how to handle him? He has such a big target area that I keep accidentally healing him

*obviously, if you keep reading there may be spoiliers below*

[Deivis]

1. choose color by which you are going to kill it (look for the most common one, as it looks like leaving one type of creature undestroyed doesn't work like in any other level).
2. unwanted colors should be kept in the area marked by red, while color you are using to kill the boss in blue areas, but I prefer right side as its larger area, let's you to find matches quicker without pile climbing up too much.
http://cloud-2.steamusercontent.com/ugc/544147366409718957/0F300E184AFA2E1A67C0A12DB85C60B1A7D2FDD7/
3. grab weapon, which can damage boss, as far as I know staves are okay, but swords don't work on it.
Keep dropping color you have chosen to blue collum, while destroying ones in red area, use weapons on boss all the time and you will make it.

[Ender]

Thanks. Extrapolating from your very helpful screenshot...
Did you use two rainbow pills to clean out the entire level before starting your attack, or was that just for making a clean screen shot?

[Deivis]

Yeah, I have lots of them, so I was using them for bestiary cards farming, but for boss stage it's unnecessary, yeah it helps, but clearing first part is more important as it saves a lot of time.