The sentence is
てなことで、修行なんてしなくとも、ただ飯を食っているだけで強くなれるんだな。
Google translates first part as "By the way". But I can't find any information how it should be parsed. I think its probably "てな こと で", but only translation of "てな" seems to be this, which doesn't match:
てな; ってな; てね; ってね (prt) (used at end of sentence) (apparently) that's what it's like, isn't it