It sounds like you are mostly having problems with getting a good build going.
Make sure you have a full bar, that means have 8 skills on your bar, in the tutorial area it does not matter that much, but as soon as you get to the "real" game you really want to play with a full bar.
Do not take both Meteor and Meteor Shower, it may seem like a good idea but in reality it doesn't work so well, both skills have a long recharge time, so it will mean that for the most part you will be down 2 skills and you don't want these two skills that cause exhaustion at the same time .
If you have to choose between the 2 skills, I would suggest going with Meteor, Meteor Shower is better when you have a coordinated group which will get the enemies set up in such a way that you will get the most use of it.
In overall play, it does not work so great because, you will be stuck casting the skill when the fight has started and mostly only get the skill done when most of the enemies have already died.
Fireball is a okay choice for most builds, good, cheap, aoe, somewhat spamable damage
Firestorm is a iffy skill, enemies like to run away from under the "shower" most of the time, running straight to you.
Use Fire Attunement , it gives you energy each time you cast a fire magic spell.
Drop Inferno, when enemies come for you, you want to get distance between you and them, not fight them toe to toe, because you will lose.
Factions is a bit limited on what you can take to do good aoe damage, because you dont have Rodgort's Invocation,Mark of Rodgort,Incendiary Bonds, Liquid Flame, or even Phoenix, which would be a good replacement for Inferno.
You also don't get Nightfall's Glowing Gaze for energy management.
You will have to work with, things like Bed of Coals,Breath of Fire, lava arrows for multihit spamable skill.
Factions doesn't really give you a good fire magic elite,because 2 of them you have to get close to the enemy to use, and the other one Mind Burn is not so great and can become a nuisance, so you will probably want Elemental Attunement.
On the Closer to the Stars part, start at the monk and go to the necro, ele and finally the mesmer, when at the mesmer flag your heroes above the slope a bit further back and switch to a flatbow and then hit the mesmer boss when you can do it without aggroing the 2 enemies walking in front of him, after you aggro the boss,hit the boss with everything you have and ingore the other enemies, if your heroes/henchmens have followed your attack, the boss should die pretty fast.
If you still need help with the mission,let me know I can run it for you and get Masters, if you wish.