Machine Gun is your 5th Weapon
OK, on-line is dead / non-existent so boosting is probably the only way to get the on-line achievements. First, just know that all Team DM kills also count towards your DM kills. So when you are working on your weapons achievements, just make sure you are in a Team DM. Here is the fastest way to get all the on-line achievements:
1. Set up a Party so you can chat while boosting
2. Choose "Xbox Live"
3. Choose "Player Match"
4. Choose "Create Match"
5. Game Type - Team DM
6. Map - Any map but Roadhouse is easy
7. Frag Limit - Set to the max of 100
8. Time Limit - None
9. Max Players - 2
10. Private - 1
This should give you and your boosting partner unlimited time to get the 25 kills with each weapon. It will take you about 2~3 rounds. I used the Roadhouse and the person getting the kills (aka "killer") sits in 1 spot (at the bar or in the basement). The other person runs to the killer, dies, hits the right-trigger to respawn quick(er), and then runs back to the "killer". While the "killer" is sitting and waiting, they can move around and gather ammo / weapons. You can trade off after each weapon achievement or just have 1 person do the full 100 kills (4 achievements). Just keep track of what weapon you use and when the achievement pops to switch to a different weapon. Use the RB/LB to cycle through the weapons.
After boosting for the weapon achievements in Team DM you should have 225 kills (25 x 9 weapons). You can go another round and get your last 25 kills in Team DM (250 total kills) here or come back later.
Next, host a Ranked Deathmatch. This time, set the frag limit to 5 (lowest). Have the other person search for your match and then go. Rinse and repeat 9 more times and this will get you your 10 Ranked matches achievement.