Guiding Bolt should use ranged spell attack so, assuming she is at ranged, she should have the same to hit chance as a fighter with 18 ST. In this case +4 (for Wisdom) and +2 for proficiency i.e. 1D20 + 6. To test this mouse over a target with a fighter (18 St) in melee and note hit chance and then do the same with shadow and it should be the same if 5E rules are being used.
BTW if you are not at range then use a touch spell.
Spells like Sacred Flame are different and require a saving throw by the target (Dex in this case), at wisdom 18 and level 4 the saving throw here would be a (8+2+4) 14; a goblin should have, as a ¼ challenge rating creature, +2 bonus (due to its Dex) so has a 40% chance of making the roll, hence she have a 60% to hit – I think Larian have played about a bit here and her chance seems less.
To be fair I have not done any testing, I am just going off my memory; please do the testing for me, I'm too lazy
