If you can’t regularly push to 800 and shield, you may have a hard time getting into a top 100 alliance.
Until then, push to 300 and get the token, pull and hope for gold… you probably need every one of them.
Find a pair that work well together (boosted or not) and try to level them up to 166. Once you have a team of 166/166/loaner, you can push to 550, etc. My first 166 was Groot. My Second was Scarlet Witch, my third was Iron Fist.
In this early stage, don’t shield, it isn’t worth it unless you are still in a noob or transitioner bracket where 500 will get you top 100. In that case, push close to 3 hours and then shield to the end. 75 HP for a 3* you need is well worth the price.
Once you get your 3* roster covered (more important than levels), you will eventually be able to play casually up to around 400-500 points and hover (that’s a good equilibrium point if you have a couple maxed 3* out there for defense). Then within 8 hours or 3 hours of the end, when you have your health packs refreshed and you’ve pulled your taco tokens and maybe pulled 5 additional health packs (never pull a taco unless you have 10 health packs refreshed) make your push to 800, and then maybe one quick fight afterwards. You’ll get your 3* cover, and shield. This is likely enough to get you top 100, and you can then try to merc your score for an alliance reward.
Once you can hit 800 a couple times in a row, try to join a top 100 alliance that has a min of 800 per PVP. When in a top 100 PVP alliance, shielding to the end to preserve your points is a must, and good etiquette. You will gain more than the 75 or 150 HP from rewards up to 800, for individual placement and for alliance placement.
Once you have a stronger roster, and you are finding your push to 800 much easier than normal, try to get a few more fights in before shielding, go for 900… etc. Then pick a good PVP where you have the boosted characters maxed, and try your first hop to 1000. It’s an amazing feeling hitting 1K for the first time… then hitting it again.
It takes time. I know in this day and age people want instant gratification, but patience is a virtue. Patience, and a good alliance, and you will LOVE this game.