Be sure that your graphics settings show attack effects so that you see the red circles on the ground when bosses are about to do some unblockable attack. Just avoid the red circles on the ground as much as possible. This is hard to do with Nightmare world bosses, but all other enemies are usually easy to learn or do.
Other general beginner recommendations:
Shift + direction key is a protected dash for most classes. The idea is to get behind the mobs and hit them in the back over and over. Also holding "S" is a block for most characters.
Make sure you have Movement +5, Attack/Casting +5, Critical Hit +5. This is achieved through a combination of armors, weapons, crystals, and food buffs.
Try to push your skill points to 1200 or more, press the "K" key, and get as many of the low cooldown (less than 8 seconds) skills as possible. Learn them. Most classes have "spammable skills", i.e., skills you can hit over and over even after the cooldown. You should max out these skills. Most classes have 1 absurd skill that does a lot of damage, typically the "spin to win" move. It typically has a high cooldown, but is usually worth getting and maxing out anyway. Use the Chenga Tome method to get to level 60, then go visit a Skill Instructor and put skill addons on your most used skills. The Attack Speed, Monster Attack, Critical Hit (for most classes), HP recovery, and Bleed add-ons are crucial.
Do the main quest lines as much as you can. Kamasylvia is when it first starts getting difficult for new players, but you can get friends to help. It is more important to get these done than it is get gear or whatever. Your starting gear based on the main questline should include: Roaring Magical Armor, Rugged Valley Helmet, Valiant Katan's Boots, Honorary Combatant's Gloves, and Asula for accessories (though obviously you should upgrade to TRI: Red Sand rings etc as you get them). You should also have the Obsidian Black Abyssal mainhand. You can use Tuvala, or the Offensive-offhand for your character. Learn to enhance and get this offhand to DUO or TRI. This should get you 170+ AP and 180+ DP.
Once you reach level 56 (and graduate from the season server) go get the Ellie/Oasis gear. That should bring you close to 200 AP and DP. Your goal at this point is to get TET boss weapons and armors then TRI "best in slot" accessories (i.e., the classical "soft cap": 245 AP + 306 DP). At some point dying to world bosses (with the exception of Vell and Garmoth) and Rift Bosses (with the exception of Ahib and Puturum) should be something that basically does not happen to you.