LogFAQs > #970998726

Garrus should be able to survive a mass effect gun sniper shot unless it's to the head

Don't think Tetrakarn reflecting headshot damage makes particular sense. So there are two possibilities. Garrus shoots more times and starts piling up bodies or he gets spooked and reevaluates. Given observation is a strong point and most buffs have some sort of visual effect I feel he might just try shooting again, which is the right move.