Ways to nerf time slow:
Limit the duration of time slow.
Limit the number of times you can use time slow.
Add a penalty after using the time slow.
Add a beneficial mechanic that can't be used during time slow.
Ways to make time slow useful:
Areas that are otherwise impossible/difficult without time slow by
a. Volume of information eg. bullet hell.
b. Size of hazard such as an explosion or beam such that you can't dodge in time normally.
c. Number of enemies to dispatch without getting overwhelmed.
State change of normally moving objeccts/hazards such as bullets turning into platforms.
Any reflex/skill based puzzle.