I've been doing missions and all, my warframe (excalibur) is rank 20ish along with my weapons.
I just completed constructing archwing and tried the quest mission and got absolutely destroyed during the outer space battle...
What should I be doing atm?

get your warframes and weapons to lvl 30, then farm and get mods to increase and make your warframe/weapons stronger, then buy inventory slots to get more weapons, lvl them all to 30 again, once you find a warframe and weapons you like, use orokin catalyst/reactor on them to allow you to double the ammount of mods you can put in them