To complete this alone, you will need a controller in one hand, and your keyboard's numpad on the other. The character you need to be playing is Ignis, your opponent does not matter, but I recommend a character with a normal hitbox like Sonico just to make it easier. You will need Alushia as your A assist, and Mukou Aoi as your B assist. Player 2 will need Mukou Aoi as their A assist.

After you set your characters up, load into the match and use P2 to damage Ignis until she has 3 gauges, then push P2 into the right corner.

After Player 2 is in the corner, do these inputs (You can view buttons in the in-game menu if you're confused about which to use that translates to the in-game button language)

E+C variationB (mash until your combo is at 9 hits), (Player 2 summon Aoi), jump, a, p1 Althea, p1 Aoi, qcfqcf B+C

(qcf = Quarter Circle Forward and is 236 using the Numeric Annotation system)

After a few attempts you should have it down and easily get the 5-Gauge Combo achievement, I hope this helped :)