OP get a full team together. Play nuketown demolition or domination and camp for kills.
best class for this:
Lsat with target finder and extended mags.
cold blood with scavnager as 2nd tier perk
Use sentry guns and lightning strikes to keep the other team in there spawn. Pick a spot on the map where you can see into the spawn and have a go at it.
Unless you are a truly great player then there is no class set up you could use to get it, all it will come down to is randomness based on how bad your opponents are, how good your connection is and how lucky you get that game, I have had about 15 20+ and around 5 25+ streaks all with different set-ups, these all just happened without me trying for them.