What would be the best monster to train on for a high lvl?

That sort of depends on what your characters are now - just about every area has some really good monsters that will give you lots of experience (no, not like the experience you get for killing the NPCs).

I've always liked the Mantraps! ;)

(1) There are several extremely valued beings in the game. These beings are well known so that I don't think I need to list them :D Well, it's unfair sometimes but XPs are XPs ...

(2) Make sure that you are Paladin when you reach guild level 5 in the Temple and get the Crusade trait. All undeads will give you twice as much XPs than normally. Shurugeon Castle or Skull Castle become bonanzas of XPs for you.

Receipe for nice level gains:

- One Scorpion Pit.
- Good storm spell.

Go crazy. ;P [Or is that cheating?]

At a high level? I wander around the desert in the lost sea area, waiting for random encounters.

Low level? Good ol' Rumphy... ;)

What about restartig the game and killing all npcs including the snake guy in valeia?