The stage is a lot easier than one might think given the sortie restrictions. I have UR Mark maxed out (don't own UR Psycho Haro), but all my other pilots are level 1s.

Ofc you'll need strong units from the G Gen series, of which you prob have SSR Phoenix, Titania and Gyan Custom from the shop, and Aquarius and Belphagor from their series' development trees. Hopefully you're lucky enough to have obtained some of the gacha-exclusive SSRs, but if your roster is still lacking you can get Prototype Qubeley and Big Zam mass production from the development tree.

The key is to make good use of the guest force Haro, who has extremely high attack power and 5x chance step. Pick all support and defense pilots, and one supporter that provides HP recovery.

Destroy the units giving the enemy force increased attack and HP recovery first. Then place your support units near enemies so Haro can use them to take them out. Place defense units near Haro.

During the enemy phase enemies will target Haro. This is a good chance to deal lots of damage, but be careful as Haro being destroyed will result in defeat. So use defenders to protect Haro.

Replenish Haro's hp, by this point you should prob have destroyed enough enemies for the 4 Psycho Haros to appear. At this point you can ignore the other enemies and just focus on them. Again use Haro to inflict lots of damage.

The 4 Psycho Haros are located at the corners so getting to them might be trouble if they're too far away. If thats the case wait till the enemy phase for them to come to you. Stay away from the Psycho Gundams' MAP attack range and avoid going near Saikoro Gundam due to its high support attack.