Why Ability score improvement is based on each class level?

Does invisibility count as “total cover”?

When 2 enemies are within melee range, do you need to disengage from each one?

Would a cleric need to make a perception check to locate an invisible creature to cast Dispel Magic?

Do magic items attuned to a familiar count against the arcanist’s total?

Can a rogue sneak attack with a thrown handaxe or spear?

Surrounded by enemies, cast levitate on myself. No opportunity attacks?