Usually, you should get at least level 250+ to kill most bosses, such as Grand Master Oberon, Grave Danger Quest, Dream Courts Quest and many more.
What you can try to do so?
Forgotten Knowledge Quest consists of 5 bosses and The Last Lore Keeper. They are located in the north of Thais, each boss give you the ability to make powerful imbuements (don't forget to click the shrine when you finish killing each of them) and when you have killed the 5 bosses, The Last Lore Keeper gives you the ability of Strike (Critical Hit) and Epiphany (Skillboost Magic Level) Powerful Imbuements. The cooldown of the last boss is 14 days
Heart Of Destruction Quest
As known as Gold Token, it is located at "The Void" (you can check where the void is using an item called Spying Eye, it might be in the North of Ankrahmun, West Svargrond or West Zao, it changes every 2 hours from the server saving)
First, you must do the access, which means kill 10 creatures of each location, then you got to kill the bosses of each location: Anomaly (Ankrahmun), Realityquake (Svargrond), Rupture (Zao); It will give you access to other 2 bosses: Eradicator and Outburst each boss gives you 1 charge, you will need 5 to enter the last boss. When you kill all of them you gain access to the last boss, The World Devour. It will use your 5 charges and you'll have to kill 5 bosses (doesn't matter which)everytime you enter the last boss, you can kill every 14 days as well.
After killing The World Devour and saying "Hi" "Worth" to Yana on Adveturer's Guide you gain de ability of the following Powerful Imbuements: Strike, Vampirism, Void, Epiphany, Lich Shroud, Reap, Dragon Hide and Scorch.
Another option you have is Ferumbras Ascendant Quest:
Find a team of 5 players and kill the bosses on each teleport: Apocalypse, Infernal, Bazir, Ashfalor, Pumin, Tafariel, Verminor.
Then you'll have access to the final battle: Ascending Ferumbras which can drop a Ferumbra's Hat
It also has a 14 days cooldown;