READ THE DESCRIPTIONS - LEIAM AS DESCRIÇÕES
About the playing:
Focus on Alaska Blood and Ice mission, starting in the easiest way.
Stick in the mission needs only, so you will have a good look about alien spaws, but xenoflowers won't have same spaws.
When you are more confident, you can play in veteran, same spaw points, but more and harder enemies.
There's no difference from normal to hardcore about difficulty.
Get used of cars explosions and turrets, no matter the level you are, they're of great use damaging aliens.
Destroy all checkpoints and you have more chance of finding legendary loots.
You will have to reach the amount of 126 millions XP to reach level 30 and most probably you will get a new level on two or three solo plays in veteran until you get level 20, when you probably have to do it for times for next one.
I hardly suggest you: DO NOT PLAY IT WITH OTHERS if you are not used to survive in normal mode, specially because hordes are made from stronger and more numerous blue aliens, so put the mission on INVITE ONLY, then you can call just someone you can trust if you want more xp in a mission.
If you find a blue xenoflower, don't use it until you get to the helicopter: normally you will get 50% more xp when you open the helicopter chest (egg?) and with a blue xenoflower you'll double the amount of xp you earn on the mission so far.
About the weapon system:
Weapons are the same level of status/configuration, changing its type (normal, rare, legendary, etc.), how many slots they have and the custom value of damage, critical, etc.
When you get a weapon, pay attention of its statistics and change it to better as possible.
Rare weapons will be your better choice here, as legendary will be surpassed for others, since you are raising levels and it changes every weapon properties.
Try to have equality on the cores (red, blue and yellow) in the weapons... and work on combinations (when you have specific cores space - that's a red slot, so don't put a yellow cores there - and when it get double or triple)...
About the abilities:
The air strike is amazing to help you when you are in danger. It is originally an attack, but you can use it as a defense system to evade and survive if you have to.
The cammo ability (saboteur) is a great help for two situations: when you're about dying (then you can run off, unless you get some shoots) and if you find an assassin side mission in the way of the mission that you don't want to try, but you can't finish the mission unless you pass (unnoticed, in this case) though that place.
Try reach the ability that makes you run 10% faster and 20% longer. It will count, I can assure you!
Work on the abilities that changes you melee attack, giving it more range/damage and it will help you when you're in a mess.
And the last, but the main ability you have to work on, second survivor let you revive with 25% of life in your first death on a mission. That's a second chance you won't dismiss.
The last survivor upgrade gives you 25% plus life energy, something you should get as soon as possible.