Do you have a full set of (awakened) draconic armor? That's probably your best bet, along with one or two of the mod's overpowered weapons. Something like RFTools' Charged Porter is probably a good precaution as well, in case the fight goes poorly.
That said, there's not really a great need to fight the Chaos Guardian unless you've quarried the entire main End island and still need more draconium.
If you need more dragon hearts and you've done the resurrection ritual so many times that you've gotten sick of it, RFTools provides a useful, if insanely overpowered, solution: summon the DE dragon one more time, punch it ten times with an RFTools syringe (so the syringe fills with dragon blood), and use that in a Dimlet Workbench to craft it into a Mob Dragon Dimlet. Incorporate that dimlet into an RFTools dimension, and when you enter the dimension, you'll find that there's a permanent swarm of 4-5 vanilla dragons around (0, 0). I highly recommend bringing along a Charged Porter and getting into the air away from the spawn platform ASAP so the dragons don't destroy destroy it (because if they do, you'll be permanently locked out of the dimension).
