2020 Update:
There was an update around October 27th 2020 that provided a fix to the XP issue. You can now obtain all of the achievements related to leveling up. There is also currently a 4x XP on the Europe servers confirmed by some other TA users.
Level 100 require ~3.8 million XP
You have a few options:
1. You form an 8 man group and ring boost on KOTH. This is the fastest solution for just XP alone. I was hearing 500k XP in 20 mins being the most optimal.
2. You form a 4 man group for 2v2 Gnasher Execution and take turns on killing. Winning all 7 rounds straight with one person obtaining all 14 kills as quick as possible takes about 5 minutes. after 10 consecutive matches you'll be earning 19,390 XP per match.
3. You and one other booster both have 2 devices each. Eliminating 2 other players requiring progress, it'll be twice as fast progression.
4. Final option, you have 4 devices yourself and you just self boost everything. Best setup on your own terms and all progress for yourself.
More than likely you'll be boosting with 3 other players taking turns. Or like myself are receiving a bit better progress from option 3. Regardless if you are going for completion you'll obtain this pretty early on your way to 10k kills for Seriously
If you have any further questions or concerns, feel free to comment below or send me a PM.
~ Slayer