Well, since you have 210k SED and 350k Event SED, it looks like you didn't get all the event slayer cards to level 100 yet, because that would bring your Event SED upto about 1.47 Million SED (210k * 7.02 = ~1.47M. So if you have them all, level them to 100 quick, if you don't have them all, well, not much you can do about that I'm afraid :-/

Regarding the gacha for slayer cards:
-NEVER EVER reset the gacha box! Not even when you completely emptied it, not even when the event is over, and also not at the start of an event, unless at the start of the event, the gachalist states there are no alpha or beta cards in the box!
-When using card reveal, ALWAYS pick the SR, even if you already have that SR but are still missing a Rare you see, unless that Rare happens to be a slayer card, then it's upto you (a Rare Slayer card will probably re-appear quick enough, but I generally grab it). The reason is simple: if you don't pick the SR because you don't need it, then it might appear again on the next SR roll, and you rather not see it again, but get the Slayer SR, so get rid of the unwanted SR from the box by picking it!)
-Remember to play the cheap N gacha as well, for the Normal slayer card. Even if it's only 1.1-1.2 points, they multiply with all the other slayer cards, so the difference is bigger than expected. (main reason I tend to hoard 100-200 N Gacha cards)

For the rest: yes, having a good mix of active big friends (to help kill your SEGs) and small friends (to help kill their SEGs) is definitely beneficial. Last round I was lacking the big friends part, and barely managed to reach the 100 SEG mark.