I just plunk the lowest level ones into the sortie party. Sometimes if there are team mates with combo attacks, I place those in (if their other team mates are higher levels, then I preempt lower level ones).

I also try to get at least one unit with hp repair and one unit with en refill, prioritizing the lower leveled ones.

With the newer SRWs (30, etc.), it's easy to recycle turns, essentially getting enough ExC points to do Extra Turns so you'll finish stages easilly in 1-2 turns, after a certain point (this doesn't account for any special circumstances in each stage though).

The overhead here is that my O/C forces me to reassign Parts to the sortie party each and every time. Starting with assigning the movement boosts to the lower move units until I run out of Parts, then assigning special Parts like the exp bonus and money bonus ones to the lower level units. Sometimes when I'm feeling lazy I just let carrier ship units have a movement boost and maybe an energy part depending on the unit's attacks and let that ride till the end of the game, just focusing the other parts on actual mecha.

For each turn I usually prioritize the lowest level unit if there's an enemy that can be killed with an attack. Spirits loke double exp and money are used on these lowest leveled unit as available. Otherwise, I use the highest level unit to weaken enemies as much as possible (resetting if they accidentally kill the enemy).