As I speak I am waiting for the last dregs of my ME to gather so that I can craft my last piece of 4-Star gear, so I decided to come here to ask what I should do when I get there.
I was considering either getting my Spine Cone up to 4-star so I can have a bomb I can use in groups or starting to gather up materials to craft the Mad Bomber gear.
Avoid fiend strata like the plague unless you have high level shadow-resist gear and a piercing sword.
Other than that play carefully. Sidestep retrode laser beams. Always block when wolvers attack regardless of where they are on screen. Be careful when you are next to jellies. Avoid the third room of arenas until you have decent damage output. Beware rocket puppies because they hurt a lot and can ruin your day. Turn your graphics settings down in Firestorm citadel.
Above all, have fun. The core awaits you. (=
-edit-
And pack a good shield. You can get away with low level/the wrong type of armor in T3, but you will be hard pressed to get by with a poor shield. (It's possible, but difficult.)
Level the spine cone up to 4* and then clean up in beast stratums. Mmm, warp dust and primal ore, bushy tails and sharp fangs! Big money on the AH.
Leave the shivermisting to the swordies, get a vortex. Electron might be better for most occasions, but graviton is fine, too. And Ash. Carry around a DA so you cant hit "your" mobs yourself, if need be.
For a start, maybe get a volcanic demo HELM and an elite bomb focus module. For armor, take something that gives you a bonus on swords. Skolver or Vog should be fine or maybe deadshot if you're going to do a lot of fsc. You can get a mad bomber suit for less status-effecty areas later, and another neat helmet to go with it.
Try the swiftstrike buckler... with proper crowd control (which you are going to do) and nitro for "emergencies", you should be able to survive anywhere (uhm... except for shadow lairs) when you get used to dodging instead of blocking. By the time you have the fsc tokens to get a crest, you'll hate missing out on the ASI "just so you can block". Maybe add a blitz needle to your repertoire - or a polaris.
Most importantly: Find a good team. One that does actual teamwork. And train with them to navigate mobs into the vortex... and to retreat into the safe nitro radius when surrounded... occasionally shieldbump mobs into the swords of the others or away from them while they are charging a weapon... stuff like that.
your first bomb tbh should be shivermist or voltaic tempest. i wish i had time to say more.