You get way more XP alone one shotting enemies 6 stages earlier from formation 4.
Fast slash is really good for taking out space enemies. Level 1 is literally do two attacks each time you attack with a downside that is worse the lower your attack speed is. The worst part is just getting it to level 3. I usually only got it to level 2 early on. Then there is also a point where you get your attack speed so high, that the flat attack speed debuff from fast slash is negligible and you farm a faster with it too, due to enemies being able to have the dodge curse.
Another tip is you can cheese space bosses with level 3 extra life and level 2 berserk. The synergy is incredible.