You just have to grind this out. It took me around 30 hours, maybe 40.
If you are going for 100% then you want to alternate your characters and guns as you go.
I would start a new characters and play the easy difficulty till about level 5, then normal after that to earn extra points for upgrading.
At first I would await until until level 15 to start doing hard.
After you have done about 50 matches, you should have your guns leveled up pretty well and you can probably start doing hard pretty comfortably around level 10 or 12 with your characters.
You may want to buy the extra DLC episode. It adds 3 more levels to the grind... but it adds not new achievements. No new content. Its a bit pricey and it didn't come up in rotation that much. I bought it, should have bought it earlier, but didn't commit to 100%ing this game until I was like 25+ hours in.
To start I would just go in order of the story in on easy, you can solo it with out too much problem. Don't worry about finding team mates. Just start. They will join as you play. But this way you can see the story and not skip the cut scenes. After that just hit quick play. 30-50% of the time you will join session already in progress.
Just use this time to level up your characters and guns.