SCL has no effect on what teams you are able to fight. It breaks down like this:
SCL - This determines the prize table that you achieve via placement and progression. This will also, in theory, determine how likely it is that you can place higher in your bracket as it is assumed that players would play in SCL appropriate levels and thus obtain scores relevant to their roster strength. However - don’t count on it. It is not at all strange to find Champed 5* players going down to lower levels to obtain high placement.
Who you can fight:
This is determind by various factors
1.) MMR - nobody really knows how this works but it is generally believed that the game takes into account your 3 highest levelled characters (included boosted characters) when searching for opponents to match you with and I believe also the number of covers (could be wrong about that) - there is a range within this value that means you can meet lower and higher ranked teams. If you level your 5* from the 255 basic level too high then it will eventually have an impact, especially if it is a bad boost week for your 3’s but if your 3* boost beyond the level of any low covered 5 then the impact should not be too severe. If you are seeing low level 5* (and reaching level 300 with a 5* does not require many covers) this is because MMR believes that you are operating within the same “range” in terms of roster strength and current score.
Your defensive team is based upon the strongest team you used last - this was changed from the previous situation to eliminate a tactic referred to as “cupcakes” which players could use to boost scores by deliberaltely leaving weak teams and then co-operating in third party chatrooms to enable them to climb off each other.
From my personal experience, I have multiple champed 4’s, the highest of which is lvl 280 and I can see teams that are at the 3* level quite often at a PvP start but as my points tally rises I will generally run into similar 4* champed teams before hitting a wall of R4G/Gamora/Medusa. It is not uncommon for my opponents to have 4* boosting over lvl400 which is out of reach of even my boosted 4*.
2.) The game can potentially match you with any other player who is playing in the time slice you choose. That includes players of a much higher level than you. However, until you obtain a certain score, your teams will normally be invisible to higher level players.
3.) Score - the game takes into account your score and the higher you get determines the amount of points you gain from winning a match, this is determined by the difference in player scores. As it is possible for a players score to have changed between the time you queue them and the time your battle is completed, the amount of points you can win may change also (upawrds or more likely downwards).
Players who “front run” PvP will be particularly juicy targets because they could be worth 50 - 60 points to a player with a low score but on retaliation, the low score player will be worth considerably less points. When you see players with scores in the mid 30’s, this normally means they are around the same score level as you currently. Higher players can eventually “break” MMR but you are unlikely to be much of a target because points that can be scored are determined by the difference in players scores - a player at 1000+ points seeing a lower scoring team it will only be worth very low points to them, whereas in reverse that high scoring player is worth mega-points.
4.) When you play. Obviously who is active at any point in your time slice will influence who you can queue. If there aren’t any players of similar roster strength for you to fight, MMR will stretch to it’s broadest reaches to try and match you. Unfortunately, this likely means that other active players with points worth pursuing are more likely to be in the upper range of your MMR and thus more difficult to beat.
)). Since I wasn’t wise as first and bought too many packs, my space is somewhat limited and I am unable to not open packs (don’t have that type of patience 
), I was thinking about selling her to free a spot and lower the PVP matches to my level (I have found a nice 3* team with Hawkeye, Ironman M40, and Dr. Strange).
)