Check out 

http://www.larkinorguide.com

 for mission info (if you don't know about it by now). If you have a wonderflask, try using it before taking on the boss (140% of your max hp), or sitting in your chair if you have a fighter's mansion or thief's den (130%), or using a deadly diavovino from the pub (120%).

Make sure you've got the best equipment for your level also, and remember that to use a level 25 weapon that you must be level 25 AND your weapon skill must also be at least 25. So if the weapon is a p/s weapon, your p/s skill must be 25. Make sure you are doing damage on the first hit by using a ranged weapon or using magic.