The easiest way imo is this:
Go to chapter 4 of the frieza event. Bring Goku SP, Vegeta SP and a fodder character. Goku SP and the fodder character should preferably be low levelled.
Use the fodder character to get all 7 dragon balls. After that, suicide him. Next, bring in Vegeta SP and get him low on HP, then switch to Goku SP. Now, get him low on HP too and activate his MA before suiciding him.
Finally, bring in Vegeta SP, activate his main and fire off a rising rush. If done correctly this should do anywhere from 2.1-2.8m damage.
If all your units are maxed out then a good replacement for chapter 4 is EX-3 but you'll have to finish off 3rd form frieza and piccolo before vegeta. Then the strategy is same as above
