The top comment here made great points and they are also good main factors to work on to improve as mercy.

I’m also bronze/silver and play mercy more as a backup to when Kiriko is unavailable or I’m playing unranked, so take my comments w a grain of salt. In saying that, I’ve been reading mercy tips and watching some videos, and some things I’ve learned aside from the top comments points, was that you can be a considerable distance (35m) from your teammates beam and guardian-angel wise, so it’s a fair distance. At that length, you have a lot of chances to find cover and look around at your surroundings to find good places to use as obstacles so opponents have a harder time spotting and picking you.

To be behind cover or any time your beam is obstructed, quickly peeping in and out every so often will keep that beam connected and allow you to maintain distance and cover.

And if there are snipers, don’t fly/superjump as much and be aware of high ground/openings in windows or perches. Some great snipers can hit some really tight angles that are basically hair-thin, so when there’s a sniper, it’s better to play more safe during that duration.

Hopefully this helps!