It's generally advised to not PvE mobs with a level higher than 2 levels from yours, especially while low level, since they are scaled to your level power (i.e. trying to fight level 18 mobs when you are only 14-15 is generally a bad idea).
You also got three things here :
- some mobs are champions, sometimes without being part of a PQ ; be careful to watch their name, wich is followed by "champion XX+".
- spiders will generally put a DoT on you, meaning they hit harder than usual mob of same level.
- Slayer will get their armor cut in half while in berserk (red) state ; be careful to detaunt regularly and use exhausting skills to avoid getting too long in red state (you will deal similar damage by using an exhausting skill while in yellow state).
I hope you'll get some clues about your struggle against some PvE content
