I’ve completed every available level in Kithgard Undeground, but I have locked some levels (2 blue, 1 yellow and one red shining). There is the screen:
How can I get access to that levels?
The ones glowing red and yellow are PVP arenas, they are unlocked after you complete certain levels in the forest world. 
Not sure about the subscriber (blue) ones though.
Im not sure either. Ive seen those in backwoods forest. Are you able to get into backwoods forest?
The blue ones are subscriber levels that are unlocked later. Unfortunately, I do not know when that level, Kithgard Apprentice, is specifically unlocked.
I am on the blue one and I need help on that one.
There are many blue ones. Please tell us the name, and further description of the problem you are encountering.
Kithgard Apprentice is the onne.
What about further description? Help us help you.
That’s a hard level: the warlock + witch combo seems rather diffivult. Or probably my tactics are totally wrong…
I started by killing the witch first, whether by chain lightning of otherwise.
I think it is hard too.
My strategy is just to get the gems and summon.