Almost all N-Slayer could at some time be obtained from N-Gacha.
Many R- and SR- Slayers marked as Alpha reapear later at (Beta) in later events.
Almost all R- and SR- slayer oly come from E-Gacha.
Very active free players might get almost all N-Slayer, around 30-40% af the R-Slayers and 5-10% of the SR-Slayers.
If you want more slayers than that you have to spend real money.
Maybe they start selling some New Gacha that offers cards from the past like past slayers but I expect those to be real money only...
Slayer girls and Medal grils are mostly give to payling players. I dont think they will change that.
Nobody know if slayer girls come back but I dont expect to get many R- or SR-slayer from past without real money.