I'm sorry but if you can't handle Horridon you will fail entirely on the next few bosses, too.
Council requires even more dps OR a group that is on top of their game in the gear they have. And even then, you will probably have trouble.
Tortos requires tanks geared enough to take on the bats, your group coordinated enough to swap to certain turtles to get them down quickly, two people (one back up) ready on the turtles, and enough dps to beat the soft enrage.
Don't even think about Magaera if you're having trouble on Horridon.
My assumption is that bosses are tuned for these item levels:
Breaker - 480-490 area
Horridon - 496+
Council - 496+
Tortos - 502+
Magaera - 502+
etc...
with Lei shen being around 517