Wall of texts incoming:
Disclaimers: Most of these are my understanding which could be wrong.
I honestly don’t know where to start to make it easy to read. I’ll try my best.
1.) Available points, floating points, etc.
The points you will earn from winning a match is determined by the difference between your score and the opponent’s score. If you win a match against someone with higher score, you will gain more than winning against someone with lower score. (I think at around 35-37 pts is when you beat someone with a similar score yours.) It doesn’t matter how strong your opponent is, just the difference between the score. On top of that, if you win, say, X pts, you opponent actually loses equal or less than X. The rate changes depending on the score. At around 400, you may lose like X/2, while at around 1k, you lose a full X pts. What it means is that, as the event progresses, there will me more pts (in someone’s pocket) in the slice.
From what you say that you can’t find a good target once you hit 400 and start to get hit a lot, that is probably because you are the leader of the pack at that time. So you become a juicy target for the other. PvP to a certain extent is a competition of roster strength. Suppose you have 400 pts with a 3* team and another person also has 400 pts but with a 4* team. You will definitely become an easier target for other people.
In general, as the event progresses, the available pts in the slice will increase. Hitting 400 within the first few hours will make you become the leader of the pack, but hitting it later, say at 1d left, won’t make you a leader or become a juicy target.
There is a term call ‘Floating point’ which means the highest score you can get without becoming a juicy target. The floating point depends on your team strength (or you roster strength), amount of time left in the event, and the slice you play. However, it doesn’t depend on what SCL you choose since the PvP system right now doesn’t separate players based on the SCL (not really make sense, but it is what it is now.)
If you want to know your float pts, you can try score as high as you can and take hits to see where you drop back down. That point is roughly where your team becomes ‘not worth the trouble’. Keep in mind that this float point also changes with time.
Will higher roster bully lower roster ? The answer is: Not really. For the most part, the game will try to match team with similar roster strength. (In my case (4* roster), I stop seeing 3* rosters once I hit around 400 pts, except the 3* rosters that also have lv 300+ 5*. Thus, leveling up 5* prematurely is a bad bad bad thing to do.)
2.) Offensive team vs. Defense team.
One aspect of the PvP is, at one point, the AI will control your team to defend your point. The team that other people see you is the last team you used and won a match with. (except for the retal which the opponent will see the team you used to beat his.) Switch to offensive when you play, switch to defensive when you walk away from the game. Some A-team (I want to say “most”, but that may be exaggerated) becomes B or even D team when AI uses it on defense. So pick your team carefully. And keep in mind that a good defensive team is NOT the team that will give you a defensive win, but rather make people want to skip you for whatever reason.
For examples, IM40+Lcap+GSBW is an excellent team for offense, but this team is horrible on defense (because instead of firing Recharge asap, the AI may choose to safe the ap for Lcap’s yellow. Plus, AI will only place Lcap’s shield tiles at random and sometimes can be easily matched away. Plus, GSBW may use her less useful red instead of Lcap. Plus, The AI have no idea how to place green tiles from GSBW’s purple right to charge up her green. Plus, etc - yeah, this team is that bad on defense.). On the other hand, IM40+Cmag+KK is substantially stronger on defense.
3.) Shield
I don’t recommend you spending any HP on shield just yet. At this stage (rank 54), I would recommend you spending HP on roster slots only. A 3* roster should be able to hit 575 for the 10 cp without shield. There may be a different tactics involving shield to hit higher score, but I still don’t think it’s worth for the resource at the moment.
Last word: One good thing about PvP is about its payment per effort. Once you get used to it, you can spend as less as 30 mins to hit 575 for 10 cp and some other stuffs. Hope this help. 