for nature raid you need grind your character to lv 70 or above and dont forget use ryen dragon roar, other tips you can use 2 character for support item heal make sure you choice character with big health like ryen, tsubaki, kaytelinth, trey and for another two you can use mirel and elly, the damage headshot combo with precition strike from elly give lot damage to enemy.
For the cg's scene you need win kiki challenge battle and the last enemy you defeated will unlock the cg scene, for defeating levia you just need equip talisman of perservation (you can obtained it from strage gold you find in manastyr amagal & research it in lab adlyn) for your character and for tsubaki only you can use the wedding ring for keeping your tp's before the battle and make sure your mp is 0 bcoz levia will absorb your mp for killing your character use curse skills.
for defeating roseris you need keep your character health above 1500 bcz roseris will targeting & give upnormal damage to your wounded character.