So, boosting this game is a bit of a nightmare due to how the ranking system works, but I'm going to try and explain it to the best of my ability.
First of all, YOUR SYSTEM LOCATION MATTERS. By changing your region in the settings of your console and restarting it upon doing so, you will be able to connect with people of a different region by choosing the "SAME REGION" option in Ranked Mode. USE THIS TO YOUR ADVANTAGE. Japan is a dead area due to this being the Xbox One version of a fighting game, and MvC:I not selling too well there as is. Japan is your best bet.
IMPORTANT NOTE: If you cannot connect, do not panic, select the option tied to the X button that reads something along the lines of "Search with less parameters" once the search fails. That will keep you in the same region, but pair you with people of a higher or lower rank, aka your boosting partners.
Rank is entirely determined by point values, and winning or losing matches will have you gain or lose points respectively. The amount of points you earn/lose is dictated by the rank you currently are the rank your opponent is as well. If you are both the same rank, you lose/gain the same amount of XP for wins/losses. But once that person levels up, this changes. You gain experience based on the rank of your opponent. If they are a lower rank, you earn about half of the experience you would if they were the same rank, and if they are two ranks, even less and so on. But if you are a lower rank and defeat a higher ranked foe, you gain WAY MORE XP and they lose a ton in the process.
TL;DR: XP GAIN IS BASED ON RANK. YOU GET LESS XP FOR BEATING PEOPLE A LOWER RANK THAN YOU. YOU ALSO GAIN MORE XP FOR BEATING HIGHER LEVELED OPPONENTS.
YOU CAN DROP DOWN IN RANK TOO. YOU ARE NOT LOCKED INTO A RANK AT ANY POINT. IF YOU LOSE ENOUGH YOU WILL DROP BACK DOWN.
This presents a problem for boosting as losing too many matches will result in you dropping down a rank, meaning your partner will be gaining XP at a slower pace.
What my boosting group did to avoid this was to always try and keep ourselves around Rank 13 or 12 and have one person make their way to Rank 10 by beating the two of us at those lower ranks constantly. Once that person gains their 10th rank achievement. Lose to the next person in line. (IT TAKES AROUND 32 VICTORIES ON SOMEONE AT RANK 10 TO GO FROM RANK 13 TO RANK 10). If a person drops below 13th rank, have them win a few matches to even out the experience gain. The main idea is to make sure that the person going for 10th rank is always beating the lower leveled boosting partners and then shares the wealth of XP to them once they reach the highest rank.
If you have any specific questions, feel free to leave a comment below and I will try and address it in the solution.