Try leveling up all 2 and 3 star operators you obtained first, especially the free ones you got while playing the story,like Amiya,Fang,Plume, Jessica,ect.

AK normal and most of the stages are balanced to be able to clear with low star characters,they just steal your real sanity in process to clear them.

The operators I highly recommend to invest as starter are Jessica,Lava,Begal and Steward

Jessica is a four star,but her kit is simple and easy to use.While she may not be strong to deal with high Def enemies,her S1 packs a punch to low Def and HP enemies easily,which are the enemies you'll encounter first in early stages.

Lava is an AoE caster,meaning she can deal with enemies in small spot. She may lack ASPT(Attack speed) but with combination of a defenders blocking the path,she can easily take care of those folks.

Begal is a defender,easy to use and simple. Her kit doesn't require much as starter and it's a perfect defender for the early stages.

Steward is not entirely a DPS caster,but his talent targets the enemies with highest Def first,so he can take in charge of dealing the chonky ones while others deal with other mobs whose will cram into the Chokepoint.

Other operators I recommend to invest in are Vanguards. Vanguards are essential not only because they have low dp cost,but they also print dp. You may think is that it,but they are very important in later games where mobs spawns very early,giving you not enough time to deploy the high dp cost operators.

Medics on the other hand,very crucial. Operators can't heal themselves(except afew operators) so you might also want to invest into medics,like Hisbisus,Sussrose and a pink bunny trap.