General

I tend to do what @therebegold said. Singularity Shock + Starforce Salvo with Planet Buster from time to time. I use Planet Buster more frequently if the boss doesn't have/add summons.
If there are summons, I place Turrent Deployment where the summons are or where I know it would be best. For example, when I Gollux I place Turrent Deployment by the portal and then I use Planet Buster to to kill the mobs on the right from time to time.

i wasn't sure what singularity shot was so i googled "jett singularity shot", and the second suggestion on google was "assd**** in space with cows up my rectum shooting"

Usually its, singutlarity shock, starforce savlo till singularity comes of cd, then singuarlity shock, starforce salvo- repeat. Now singularity shock does require you to be in melee range for it to hit the boss. For instance when facing hilla you could stand at a distance and just spam starforce savlo, but she will go down much quicker with a combination of singularity shock and starforce salvo.

Planet buster is usually busted out only when the boss summons adds.

From what I understand it's dependent on the boss, like for Gollux you just use Salvo but for Czak you use a combo of Salvo and Singularity. I'm not entirely sure on the reasoning, something to do with how close you are to the boss maybe?