Need help: More than beginner, less than expert

So I started playing this game just before Ultimatum League and really enjoy it. I read a couple of beginner guides to get the basics and got some tips from in-game chat. Now I feel like I'm hitting a wall that some better gear and possibly some tweaks to my passive tree could get me past. I've currently just started doing maps and I'm able to clear tier 2 and 3 maps fairly safely, but I feel like I'll start dying a lot on the next tier and beyond.

My current build is an elementalist using firestorm as my main skill with frostbomb as a secondary/support skill. I try to hit mobs with frostbomb to chill/freeze them, then kill them with firestorm. The playstyle for this build is very fun and I love using multi-elementalist builds, so I'm reluctant to remove the cold skill aspect completely.

Some questions:

What stats should I be looking out for when considering gear upgrades?

Are there uniques that would enhance my playstyle?

What major mistakes have I made on my passive tree if any? How could I make my skill tree more effective for this playstyle?

I don't anticipate being able to play this character all the way to level 100 or even to the top tier endgame content but I think I can go a lot further with some small tweaks and gear upgrades.

Thanks in advance for your feedback and input.