Ah good, never came round to make a topic like this myself =)
The Holy Grail challenge isn't completing stage 12 using only barracks though,
it is completing IRON stage 12 using only barracks.
Also, the no stars challenge has been confirmed to be possible for
non premium levels. You might say completing the game means
completing the first twelve stages... but that could be made clearer
Oh and there's another playermade, possible challenge, called
'The Holy Arrow' which is completing IRON level 12 with only archer towers ^^
+ we have an unnamed challenge: completing stages with only reinforcements
and rain of fire. Completing all stages this way surely isn't possible,
but stage 2 can be completed this way, for example.
Hereby I name this challenge... Spells only