Subs or bosses are very, even extremely uneven difficulty, maybe meant for grps, I could do some in high level areas and couldn't in mid level ones. They either had too high regen or dealt out way too much damage (like you reach for your skill key and lay flat on floor in instant).
Can't really talk about sub or a boss here, there's many of them and can spawn anywhere on any cave floor it seems, can be hard as nails or easy, often spawn literally on your back, like if you triggered the spawn or maybe you just happen to be there when the boss spawns (but I think you trigger it). A map you've been hunting on for days and you think you know how it goes there turns into a sub fest next day, you run into a sub every few steps all of sudden.
And I have doubts about those server counts, some of that count are likely some bots, anybody who logged on any korean server may have seen those coming online after server reboots, I recall seeing them on the official korean mir2 years ago and I've seen them here. Was told it is 'Ai' which I take is artificial intelligence, that is robots. But you don't see them in game otherwise than at reboots, at least I have no idea where they go. In game it definitely doesn't look like there's that many players that server count says.
BTW I don't think those robots are a player char moved by macro like we know bots (like some players make extra chars and control them by macro), my guess is it is some 'company' that plants them on gaming servers, probably not just mir2, because they all move in unison when they log on, they run in a line just under bw sz and they all 'vanish' at a given spot on map (bottom right below sz) where there is nothing, they make sound like they RTed but I don't think its that (because you can RT anyplace, why first run to some 'magic spot').