I started back in December of 2018, and so far I'm really enjoying this game. Now most of my medals are 7 star, with the rest being 6 star medals, I've supernova-ed and supernova plus-ed a few medals too at level 303. And I still can't seem to stand a chance in the "Expert" Event quests and stuff, once the enemy level hits 1500, it's like nothing matters. Anybody have any tips on getting stronger, or ideal builds they could share? I'm a casual-hardcore player, but I never did and probably never will buy a VIP pass. Any pointers you guys can give would be really appreciated  Thanks in advance

Well, the events right now have prizes that aren't extremely necessary, so don't beat yourself up too much if you can't complete them. The expert level stuff usually requires extremely good medals with very specific traits. If you don't have ground def -60, for example, you won't do much against a lv. 5000 enemy. The reason for that is because your strength has to overpower their defense, and the only ways to do that are to have a high strength stat (primarily from your supernova medals), use all available buffs/debuffs (atk up for you, def down for them), and have either ground/air def -60 (depending on which type you're facing). If you're able to hit all of those points, it's hard to say why you wouldn't be completing the events.

As for setup advice, you'll usually want to use Starlight, Stroke of Midnight, or Fairy Stars (make sure they're at or around lv. 35). Your buffer goes in the first slot, preferably something like SN KH3 Kairi or SN Xion. After that you'll want to follow up with damage-dealing medals that make up for any buffs/debuffs you haven't already covered (for example, the 2 I mentioned only do +12 PSM atk). You don't have to match your buffer medal to the slot multiplier (you don't have to put a power medal in a power slot, in other words), but you want to try and make sure all of your damage-dealing medals do match the keyblade slots. Your most powerful medal should go in your pet slot as that is the highest multiplier on any keyblade you use. Oh, and make sure you have a high subslot value (using those 7-star medals you mentioned).

Thanks Hawk222, I think my biggest problem was that I wasn't using buffing and debuffing medals, that and I was wasting my time trying to get skills like Max Gauge+2, and and STR & DEF stat boosts, when I should've been retrying for Def-% skills. I really appreciate the help, looks like a have some work ahead of me

To be fair, STR traits are still worthwhile, but only the +1000 and +2000 traits. Anything lower than that (which comes from pet training) is worthless. It's a good idea to mix the ground/aerial def traits with STR traits, as well as Extra Attack traits (and raid traits if you plan to use the medal for raiding).
DEF traits are the least useful, with HP traits being a close second.