I'm really interested in it, but wave based defense games don't hold my interest for very long if they are too easy. They're either too easy throughout, or they don't become challenging at all until many hours into a game. Can someone fill me in?

I wouldn't go into Trenched by myself. Aspects of it might be too overwhelming for one person. Most stages have you defending multiple positions. However, with a full 4 person party the game is too easy. It's the unfortunate design of Trenched. 3 people seem to be the golden number, but the host has no control over lobbies. Thus, if you allow people into your game, you will get a full party most of the time.

It's an entertaining game nonetheless, i highly recommend it. The game doesn't implore teamwork too much, I played with strangers most of the time and it turned out well.

I've beaten all but the last level by myself, and the reason I haven't beat the last level is because I haven't tried yet. There are some hard parts, especially the second to last level but it is not unreasonable. 

The missions are easy enough to get through once you start to see some good gear and if you follow the mission advice on what gear to bring. It's a little more challenging to get gold medals, especially if you play a mission solo. If you're planning to play it, I'd start playing it right away. A lot of people have gotten some good loot, meaning most missions are a cakewalk.