Normally one of the team members is a tank (wears high ranged defence equipment) and always makes sure the boss goes after them. The other team members wear karils to reduce damage done by the mage.
Just watch the guardians of fate a few times to learn what to do. A sucider just attacks supreme and prime so they're not attacking the mage who is soloing rex.
Your stats are good but I think a few more defence levels woukd help you stay a bit longer. It is possible to solo Rex but you will need a suicider to get Supreme and Prime off you.
I've ranged the barrows and it was actually quite succesful. I used enchanted diamond bolts. This guide explains what to do. http://forum.tip.it/viewtopic.php?t=688196