Elin is easy enough to get. It's all about level requirements;

Player needs to be 20 to get her.. or was it 25... lets say 25 to be safe. As well as a rope ladder.

Then you need to be lvl 30 to get her gun. (have a lvl 60 + team)

And then Elin herself needs to be 40 to get her chip. (have a lvl 80+ team)

Her death is not that particularly hard... I would say about medium (scale 1 being easy and 10 being hard.. about a 7-8ish because of debuff skills) but it all depends on the team (recommend a rebirth team)... only "hard" thing I would say about it is that you need 6 black bombs and a submarine to get under water.

As per Charlotte and Eva. Charlotte is hardly used as a pet cause of her low attack plus you can't equip her with all human gear or brooch since she's seen as an animal pet. So her attack will stay weak unless you have money to invest in her. She grows in wis and agi. Her hp SUCKS so best to have an earth stone wall if you want to properly lvl her. (she can busrt but you get at most 5k exp doing it)

Eva is good but same as charlotte you should try and scroll her at 20 if you want her to have decent HP or do good damage because she grows in wis and speed also same with Angela. All 3 seem to like WIS and AGI then INT

Since you are asking for Rainy's opinion I won't say anything more.