What type of character are you playing?
A warrior can use a shadow potion and then spin attack close to an opponent to do damage without turning visible (right click on the ground, not an opponent), making any fight easy.
For a mage, Meteorstrike can be cast very quickly and has a bit of a stunning effect. Survivor's Curse cast repeatedly followed by any elemental attack works well, as does Blind + Hell Spikes.
Any character can use a hit and run tactic with a bow, or Deadly Disks, etc. Burning Wall can be used either to slow down (and damage) a following opponent, or to surround an opponent (or group) to do even more damage.
The techniques in the following topic (the more popular of which are mentioned above) will work for any strong opponent, though it may contain spoilers.
A trip to a few merchants looking for better weapons or armour may help.
If you quick save before running your mouse over an item/chest/merchant or hitting the alt key, you can identify the item or trade with the merchant, then quick load to get different stats or inventory. This can be done repeatedly to get the best stats or specific bonuses on an item (save after finding good candidates) or looking for specific items (such as spell books or charms) from merchants. Don't waste too much time with this, though, it really isn't necessary to get the best possible equipment.
Welcome to the forum.