Knocking targets prone limits the targets saving throw to "1" making sacred flame always hit. Combine that with a mage and grease or ray of frost to maximize damage synergies.

At level 1-4 the highest we can get the DC is 14. From what I can research, caster Difficulty Check for landing spells that have a saving throw is (8 + your Spellcasting Ability modifier + your Proficiency Bonus). Highest we can get a stat is +4 and highest proficiency bonus is +2 until level 5 (+3), level 9 (+4), level 13 (+5), and level 17 (+6). So (8 + 4 + 2 = 14), but typically we will be playing at a DC of 13 for most casters right now. Meaning any target has to roll a 13 or higher with stat bonuses. If the target has +2 in dex, then they only have to roll an 11.

If we were level 9, now our DC would be 15-16, making it more likely to get a spell like sacred flame to land....anyway, that isn't happening right now, so we need to reduce the saving throw on the targets. Best way is prone and/or disadvantage.

Bane of course, or to cause disadvantage.

I was trying to test just now if things like shocking grasp (no reactions) or sleep helped sacred flame land.

Shocking grasp did not reduce saving throws.
Sleep did reduce save to 1.
Hold Person reduces save to 1.
Halt did not reduce saving throws.
Invisibility on caster did not reduce saving throws.
Threaten by being in melee range did not (of course, only on attacker's ranged attacks and not on defender's roll).
Fog cloud did not and I couldn't even cast sacred flame on the target because I was always "too far" to see the target.
I did not have a raw blind spell to test to see if that reduces saving throws, but fairly certain it will not, just reduces attacker's change to hit.

"pretty sure" bless and guidance do not help sacred flame, because that gives you advantage as an attacker and the saving throw is for the defender. So same with invoke duplicity.