Here i will introduce Cronous boss guide for you.if you are interested in it,please read it up.Hope the article can help you. I believe you will learn something from it.

Beginning in a place called "Altar of Lava" how to get there: use maelle in castle cronous and teleport to ketters hell. once in ketters hell, open your mini map, by pressing V, and locate the little exit mark on the bottom of ketters.. that is the portal you want for the 1st boss room.

upon entry please make note if there is a counter.. and what i mean by this is.. there should be a number, as high as "88" up in the left corner near where your name is... now you need to kill all "88" monster to make the boss appear.. but, there is a trick to this counter.. if the counter starts climbing again.. for example, the counter says "65" one second, then the next second it raises again to "72", stop killing, move to the ledges in that aera where the mobs cant hit you.. and wait for the counter to reach "88" again before killing...

once the mob off "88" is defeated. the boss will spawn with his baddies..and then a time counter will start, you have 10 minutes

Boss name : shredder
Level limit: below 50 (if i remember correctly)
Time limit: 10 minutes, if you pass the time limit shredder will just dissapear and you wont get anything

Boss Area: Ketters H.ell
now to the second boss area.. AoB, Altar of Blood

well ok how to get here.. there are a few ways, but the FASTEST way is.. go from Sitis Terra, use maelle and transmit to tarra, then open your mini map and find the exit to mont 4f.. once inside, if your facing straight forward and cam angle as well, walk forward go left, the left, then left, and you will see the next portal.. enter the portal, and welcome to AoB..

now for Idra... muaa haa haaaa >=]
now you will probably notice again, there is a counter.. this is the way EVERY boss works (as far as i can remember).. so do the same stuff you did you the 1st boss, and Idra and her minions will appear.. simply defeat her within the time limit to collect your rewards

Boss name: Idra
Level Limit: Above lvl 50 (not sure what level it caps at)
Time limit: 10 minutes
Boss location: Montanus 4F

and now for boss #3..