What kind of tactic can Wurzzag play for free? An orc tactic that is not an epic spell or only orc spells (that aren't epic spells)?

Wurrzag allows you to play Orc tactics that don't have the "Epic Spell" trait for free. "Epic Spell" and "Spell" are two separate traits, one isn't a subsection of the other.

And since this came up elsewhere: "For free" is the same as "at no cost", which is explained in the FAQ.

Great, thanks for the reply!

How does Wurrzag works with Smash-Go-Boom! ??? Am I allowed to fill "X" with the number I want for free? Or is "X" considered as "0".

X will be 0. You won't blow up 10 developments for free, you'll blow up 0 for a cost of 0.