Sagitta Ptolemy is a single-target DPS centered on delayed execution damage from Golden Arrow, it deals solid upfront damage and then threatens a large delayed hit on the target's third action turn. Hidden Archer adds survivability through chance-based stealth after acting, which helps him stay alive long enough for the delayed damage to trigger.
The main weakness is reliability, since the arrow disappears if Ptolemy dies and the target has multiple turns to cleanse, shield, or end the fight before the execution hit becomes decisive.
Ptolemy is a medium beginner pick for focused single-target pressure and can punish unprepared enemies in early progression. Even so, he is still a B-rank transitional unit, and for efficient account growth he should eventually be replaced by stronger saints.