I know the basic way to recruit for your class is your stats and/or skill level in certain things but is there an easier or faster way to recruit than that because that way seems very costly and time consuming?
If you build support with the units your trying to recruit it speeds up the process, ask units who you want to join for mission assistance to build support and try inviting them for tea or in the mess hall and stuff like that.
Also make sure your getting faculty training in what you need to recruit them as well. Try and focus on about 2 students you want to recruit because realistically your not gonna get more than that
Also, if your female Byleth you can get sylvain on the spot.
I don't know, I wouldn't consider anything here spoilers, especially since nothing in the story is mentioned, but better safe than sorry. All this is is a guide to recruit characters in 3H.
Only way to do it is by grinding stats, support, and weapon mastery HARD. I have a weird thing with recruiting all the characters in every FE game every play through, so I just BARELY recruited everyone until I couldn't anymore. I did many training maps with Byleth all by themselves to grind up weapon ranks and stats. I also did a lot of faculty training and dining to grind supports and weapon ranks. Buying all the gifts from the merchants each month is also important to build support. It's an absolute grind, let me tell you. Do keep in mind, however, that there are a couple characters from each house you simply cannot recruit to the other houses. But that's what I did to get them all. Hope this was helpful. Good luck!