You gain levels as a coach by completing league matches against the AI or
ranked league matches against players. The game was patched so that you no longer gain any coach experience from single player or non-ranked league matches against players.
You need a total of 706 experience to reach level 20. You gain 10 experience per game you complete (no forfeiting) and an additional 3 experience if you win. This means you need a minimum of 55 matches won to reach coach level 20. If you are good you can win a match against the AI in 30 minutes but the difficulty increases the better your team gets. Still, looking at a minimum of 28 hours isn't really promising. Fortunately, there is an easier method.
This can be boosted in ranked leave matches. To do so you need at least one other player. You need to create a new team for it to be eligible for the ranked league. Once you have a fresh team (you can use the automated roster generation), join a multilayer league and look for the sponsored NAF Ranked league with, at the time of writing, 8000+ players. It is by far the largest league and should be easy to spot. Now all you need to do is join the free competition and coordinate with your partner.
The matchmaking system in Blood Bowl is on a five minute timer. Even if there are enough people, it will only start matches in a wave. So don't panic if your match won't start immediately. As soon as you select
to
"Find Opponent" it will also tell you the remaining time for the wave so you know when your match will start.
The following section was written when a second, non-free competition was available. Currently there is only the free ladder. Should there be a second ladder again, feel free to use the striken-though paragraphs to half the time required to boost this achievement.
You can use this to your advantage. As you joined the free competition, you may have noticed the 2015 Fall Ladder competition with a 20 Cyan entry fee. Once you played a few matches and have the coin you can create a second team and enter this competition. The reason is that the timer for both competitions are slightly off (15 seconds) and, thus, you could start two games per wave if you are quick enough. All you need to do it quickly hit again once you are back on that screen after your first match ended as starting a match removes you from all queues.As soon as you are matched up you can select the resources you want to buy for the match and, fortunately, you can also hit
to concede. Whoever concedes gets no points but the boosting partner will earn 13. Using this strategy you will get 13 XP every 10 minutes or roughly 78 an hour. Using this method you should be able to get the achievement in about 10 hours.
If you have set yourself up with a second team and are fast enough to re-queue before the wave for the Fall Ladder launches, you can each get one win per wave, that is, 5 minutes. This bring down the time needed to boost this achievement to just short of the five hour mark (12 wins an hour).