Hey man I have a xbox if you want me to help you. And if your still trying to get your nuclear, I'd recommend the M27 with grip, quik draw. B23R w/fastmag, shock charge (only for camping) a betty. ghost, engineer, cold blooded or scavenger.
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.
Mk48 -> Silenced, TF
Game mode: Domination
I did stick around the fountain in the middle where the B flag is and just picked off people as they came at me.