Sadly, especially for Lae'zel, you really cannot play the class "as intended" as others have mentioned, there's that special blend of homebrew in BG3 from Larion, which ultimately over-rewards you for backstabbing.
giving Lae'zel any source of light (theres a necklace that gives you dancing lights as a bonus action) because she has no dark vision, but also bringing her from the front of the target (as you would think a warrior would fight toe to toe) you'll find her chance of success marker raise from usually around 55-65% in the front to 80-95% from behind (even higher if you can successfully be in "hiding" before you attack them from behind - see the red vision cones that appear in a battle, which way people are facing, etc.
Shadowheart can run into this problem as well with her melee, and depending on what kind of character you're playing (rogue or warrior for example) eventually you'll run out of "backstab" area with too many melee targets
Using shove on targets can help with both this issue, so the target moves away from lae'zel after she attacks from behind, so shadowheart can walk in behind and do the same. Pushing someone away also allows you to break away from a target's melee range without incurring attacks of opportunity from them.
another way to avoid attacks of opportunity, or to position yourself behind someone without them hitting you is to use jump, jump uses a bonus action but again, if you can just leap behind your target, your chance of hitting will drastically increase.
the same can be said for ANY range skill (arrows or magic) using them from higher up (even just standing on a box, literally) will raise your chance of hitting the target drastically. Elevation = Success, and the reverse for being below targets, its hard to shoot anything, even magic, upwards.
The only other really big thing I've noticed that messes up my success is the "threatened" status, which is nasty for casters, but particularly frustrating for melee fighters because of course they're threatened, they're fighting in melee. But I digress, if you are threatened (a hostile is within arm's reach) you are, in theory, so scared or in peril or whatever that you mess up your spells and attacks, pushing and jumping are useful here too
Trying to capitalize on shared inititive is key too. If Lae'zel and you are going at the same time, and she has a 30% chance to successfully hit because she's in the dark, and you can cast light. Cast light first, then make her attack, or inversely say you are trying to cast eldritch blast but you're threatened. Have Lae'zel push them away so you lose your threatened status and have a higher chance of hitting the eldritch blast.
Good luck!!! May the odds be ever in your favour!
