Get 20k likes and like back 3k: There is 2 ways to do this, be in a group similar to a mini Likefest, or go solo. Seen it done both ways. Group way get a group of 25 or so and run Likealizer/RockStar/Liknado combo or maybe 2 depending on participation. Solo run Rockstars/Likenados all day until you make 20k, usually 3 sets of each, maybe more.
The hardest part isn't 20k likes received, it is the 3k like returned, return like use this I'VE NOT Liked BACK today for checking for # of return like you have done you use this link Who I'VE Liked Back Today , once you get 3k you are done with that part.