What does exactly なんだり here mean?
I understand he's saying "We meet many monsters in our own way and it was terrible" but what does なんだり actually add?
It's 遭遇したり何だり(で)大変だったしな。
As you may know, AたりBたり is used to list multiple actions or events, meaning "do things like A and B".
Here なに means "something" or "other things".
「Aたり何だり」 "do things like A", "do A and so on" is not a common phrase. You may be more familiar with:
「AとかBとか」→「Aとかなんとか」(things like A)
「AやらBやら」→「Aやらなんやら」
「AだかBだか」→「Aだかなんだか」
「AなりBなり」→「Aなりなんなり」
「AかBか」→「Aかなにか」(A or something)
what does なんだり actually add?
It adds that they did other things (had other troubles) than 色んなモンスターに遭遇する.