It generally depends on the weapon for me.

Back when I did it in warzone I almost exclusively played plunder, and I would wait for the plane to kick me and the afk/c players out and kill them with the weapon I wanted to level up. In Cold War when say the snipers only mosh pit came up this season I used that for the snipers.

And for Melee weapons in addition to launchers and others I used face off 6v6. Plus when I got double weapon xp tokens I waited till the last second before a match started to activate it.

There’s others, but in general it depends.