What’s the best way to get 2.5 million Rising KO?

First of all you will need:
- SSJ Goku Red 1500 level
- Krillin at a low level
- Android 19 Blue low level
To inflict the 1 million of damage (you can make also 4 million of damage) you need to use first Krillin, he need to die so the ability of goku will be upgrade. Then use Android 19 main ability (this will give you a green card) once the android 19 was defeated use the green card with goku that will upgrade his damage, Finally make the rising rush.
Also...
- Collect all the dragon balls with Krillin and android 19.
- Try to keep the super attack card to make the rising rush.

*You can use gohan SP Yellow instead of the Goku SSJ and use picolo instead of krillin (if you do this make the same process explained before, but with this characters) .

From the new Event Guide:

Picking the proper Element counter for your active fighter: +50% damage
Weaker opponent takes more damage: up to +200% damage
Comeback bonus when downed fighters: +25% damage per downed fighter
Special Arts (blue cards) in your card selection: +20% damage per Special
Equipments that boost damage
Equipments or z abilities that boost base stats

https://dblegends.gamepress.gg/invincible-android-18-event-guide#rr

the easiest way to get 2,5 million damage is only by getting at least 2 special move or more and the element weakness of enemy character. you can get special move art by using main ability of sparking first form frieza, sparking green ssj goku, etc. if you're working on android 18 event missions I suggest you to use my team set like in this picture then choose android 18 mission on chapter 4 and the last put the low level character on the first to get dragon ball for rising rush. after the dragon balls is complete change to goku and frieza to use their main ability to get the 2 special move after that use your rising rush on the red android 17 using frieza. by using this way I can get over 3 millions damage and more if you soul boost and equip the character

You can Activate you're strike Cards with the Dragonball & then side step before the attack connects to negate you're own attack but you'd still get the dragon ball.

as for the Ki blasts, you have to time it right & use them when they use theirs, or if you use your ki blast & they somehow counter it with something else you can get the dragon ball

SSJ Goku Red, & one of the earliest battle.

https://www.youtube.com/watch?v=Qsxt1BYlFVY

https://www.youtube.com/watch?v=12IzUXRhhI8

Infact there are two videos explaining this. Hope that helps