The player opponent matchmaking system is based on your amount of Victory Points and in certain cases also on your Headquarter level.
If you are on the lower end of VPs there is a higher chance that you will be matched based on VP+HQ level. By taking the Headquarter level into account, the system tries to make sure that you and your opponent are roughly on a similar ability level (in terms of offense & defense).
If you are on the higher end of VPs, matchmaking will be based only on VP. That is because after a certain VP score, players are usually anyways of a similar HQ level or even maxed out.
That being said, there are of course exceptions. So it can still happen that you’re being matched with a player who is a few HQ levels above you, but it shouldn’t be the norm.
If an opponent seems unbeatable or doesn’t render good loot, you can change the opponent after a certain time.
Once a player’s base has been on your map for a while, a small "!" next to the avatar will appear. It indicates that the " Find new opponent" option is now available. This option allows you to exchange the existing player against a new opponent.
Please keep in mind though, that there is no guarantee that you will get an easier opponent!