First of all, you can do this in versus mode with two controllers. Now go to customize and equip the the Auto block gem for any character(I used Zangief!). Now in Versus mode, make a 7 round match, press start on the 2nd controller, put it on the other team.
Now make sure you put Zangief for your second fighter, otherwise this method wont work properly! 2nd player's team can be any two fighters. Now put your health to 25% and the 2nd player's team to 0%. Once you begin the match, go Pandora mode(Requires 25% health and press Down Down mp and mk together) and you have about 7 seconds before you lose, but you also gain infinite meter too. Wait until you have 3 full bars on your meter before you do anything!!! Now with the second controller just walk up to the first player and attack him 3 times(each hit counts as a activation, so you get 3 activations per round), & the gem should kick in every time you get hit. So when you use all 3 bars, just hit the second player once and you will also get a Pandora Finish that is also required for 2 other achievements!!
If you did everything right, u should have: 12 Assist gem activations, 4 Pandora activations, and 4 Pandora finishes!!
So by using this method You will get achievements for: Activating 100 Assist Gems, Activate Pandora 500 times, Finish a round with Pandora 300 times, & an other achievements you can squeeze out!!!
I kno this is long, but its a really easy method to do. If anyone has any problems with this guide, just let me kno!!!