Have you seen someone take on a boss monster you've struggled with and take it apart in no time, and you don't understand how? Using counter-attacks is the way to do it.
Lost Ark is an MMOARPG. This means that the game is heavily reliant on player skill, rather than mere stats. Due to that, some encounters can be more difficult than others if all the tools at the player's disposal are not used effectively. The most important tool, when facing boss-class monsters is abilities with a counter-attack option.
No matter what class you're playing, due to the sheer size of Boss monsters in Lost Ark, you will find yourself getting uncomfortably close to them, giving you no time to simply dodge some of their dash, charge, ramming, or other abilities with similar effects. Every boss monster, however, glows while charging these abilities, as a signal to players to prepare themselves for a counter-attack. In Lost Ark, "counter-attack" is a tag on certain abilities, rather than ability itself.
This is the moment when you use your counter-attack ability to turn the boss' Crowd Control against it and deal some free damage.
Every class in the game has at least one counter-attack ability, even Bard. All you have to do is find which ones have this effect, unlock and equip them.
It must be noted that some abilities, on certain classes such as Paladin, are more suited for doing this and are easier to execute. However, in the interest of balance, even if you're playing a squishy Reaper you still have the option of doing so.