Balance options that make it better, but not too good (these are alternatives added to the existing spell):
1.) Make the duration 3 rounds (concentration).
2.) Make the casting time a bonus action, but don't allow it to be cast if you've moved this turn, and have it end if you move.
3.) Make the casting time a reaction based upon being attacked - have it last until the start of your next turn.
4.) As a reaction, you inflict (ability score modifier) radiant damage on an adjacent target that misses you with an attack.
5.) Damage resisted is added to the damage inflicted on one target of your choice that is damaged by the next spell you cast before the end of your next turn.
6.) A target that misses you with a melee weapon must make a Wisdom save versus your spell DC or their weapon flies from their grasp and travels (d4-1) spaces in a random (d8) direction (1 forward, 2 forward and right diagonally, 3 right, etc...)
If it's an ambush, the casters are going to have advantage anyway from being hidden.Regarding True Strike, it seems to me the cantrip is very useful as an opener when the caster has set up an ambush. All of the high elf scouts in my campaign have it and it makes them truly dangerous if they are hiding in the forest in an ambuscade. At low level combat, gaining advantage on a first strike can be absolutely devastating. PCs can use it situationally when they are about to kick in a door.
If it's an ambush, the casters are going to have advantage anyway from being hidden.
Not if they are spotted. I'd still take the True Strike. It doesn't cost anything to cast pre-battle.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.