Here's what you do after the campaign:
- Get your good Uniques,
- Fix your Blessings,
- Slowly start pushing Corruption, do the Forgotten Knights faction,
- Work towards good 3+ LP weapons and 2+ LP armor.
In boss fights, leverage your mobility; stay close to the boss and spin around it. Many bosses have directional attacks that will target your last known location, but have a windup animation, so you'll already be gone from there.
Since targeted attacks are "solved" by always moving around, ground/arena effects and delayed explosions are the things you have to watch out for. Remember Volatile Reversal when you think you're dead in the center of an incoming Orobyss meteor.
Heorot fights: Face him towards a wall, and only do small back-and-forth movements in a 180° cone in his front. If he charges away from you while Blizzard is up, you will need all your potions to survive and catch him with Lunge.
Lagon and Mountain Beneath fights: Count your blessings. Both arena stages have few enemies you can leech health from, so you'll be sitting ducks. Use Volatile Reversal before potions to regain health, but remember how much health you had 4 seconds ago.
All other bosses I've faced so far did not prove to be an issue. Some of them, even Shades and Harbingers, will just fold to an angry guy continuously spinning around them.