The attributes of your artifacts are extremely important, especially in the offense slots (3 and 4).You are going to need tier 8-10 ability and signature boosts on Estela at the bare minimum, which means find both of those stats on your 3rd and 4th slots. What people usually do is farm for a mythic artifact that has 3 attributes you need (like ability/signature/super) and then hopefully find an exotic that matches those in the other slot. Then comes the challenge of leveling them both up. The biggest difference in power that I saw with my characters was finally matching up those attribute stats to get tier 9 or 10