Fighting higher level titans gives better loot tiers: Titans: Grades, Loot Tiers, and You
With your roster, you can be killing 7* titans easily with a stronger alliance.
Using the formula in that linked topic:
Killing a 5* titan at A+ grade = 5 + 3 = loot tier 8
Killing a 7* titan at B grade = 7 + 1 = loot tier 8
See, it rounds out, and if you perform better against that titan with the aid of a level up or a refill, you can get better stuff.
As per the topic I linked above, B grade is the minimum everyone gets if they deal 3.3% damage relative to the titans health. With 6 hits that is easily achievable.
You only get C grade if you deal less than that percentage of damage. It can happen if the titan is killed too quickly (which means that stronger titans are likely on the way) or if you got seriously unlucky.