This advice is for everyone, of any player level, with any set of characters. I play two accounts, one with mostly 6* and 5* units, and one where I only use 2* or 3* despite having rarer units (just to test how far it can go). There are some differences and some major similarities to how I put the teams together, and I'd like to share that since I have actually not seen anyone else organize their helper squads in the best way.

Choosing the knights
I won't argue with what units are chosen. Especially with the helper squad, it's easiest to just use highest to lowest for either power or attack. Some people like to make sure they have at least one of each attack type (slice, hit, pierce, magic) in each team, and that helps if your teams are a bit weak for the mission you're attempting, or you could see what kind of enemies are most common and fill your team with girls that are strong against those enemies. Once you have more units, you might try paying more attention to ability combinations, like... Lily Rubellum gives bonus damage on the third turn, and Moonvine gives extra skill activation rate on third turn, so they work well together. Or group up girls who have a large bonus to solar blast, and use that whenever they fight. Also, as a general rule, more stars = stronger, so don't worry too much about type balance unless you're choosing between units of equal strength.

Speed
This is important and not important at the same time. Generally, it matters when there are a lot of pest squares: you'll want to clear those ASAP, so make sure you have fast squads for those paths. Otherwise, it usually doesn't matter their speed, except that you'll want slower squads on closer to the end and faster squads further from the end. If you find that one squad generally makes it to the end before another squad can reach enemies, only then do you really want to mess with speeds.

Order within a squad
This is the one that I really wanted to get to. Characters attack in order, so shuffle your units around so aoe goes first and single-target goes last.

The first two characters hit 3 targets, the third character hits two, the fourth character attacks multiple times so can hit multiple targets or one single target, and the last character hits one target. AOE characters going first can weaken enemies before single-target characters finish them off. If you have it in the opposite order, single-target skills can finish off one of the enemies, halving the damage your aoe spells do.

So please put Lavender at the front of your squad, and not the end. :P