Generally; how do people go about PvP?
It all depends on the server.
RPGs tend to have restrictions on PVP, like needing to turn it on, level/nation requirements, etc.
If you want an instant battle, any TDM/DM server will do.
Most if not all servers that include fighting other players use server sided add-ons. Though it's not required, it's recommended.
A client to interact?
You must be referring to the Dueling server.
It's a lot more complicated than just fighting an opponent. Trust me.
Boundaries?
Brick walls and a ceiling will work just fine, as long as you don't include vehicles. (things like horses and cannons are okay.)
How to deal with trolls?
Just make the spawns invisible, and make a lot of them. Avoid 'class selectors' and you won't have many spawn killing issues. Players have an invincibility time when they first spawn.
If I wanted to host a PvP game, how should I go about it?
The server sided part of the Dueling mod is private. You can't host that.
As for slayer, just download the add-on, enable it. When you start the game, and after you've spawned, press Esc and click Minigames.
Click the slayer button and all the settings are in there.