Best way I've found is to make a square 6x6 or bigger of double crops, plant your seed in one corner, and let it spread all the way across to the opposite corner, by the time it gets there it should have jumped a handful of levels. A 111 seed should easily be close to 555 or better by the other end. It's best to analyze all the seeds on the far edge though incase the highest genome seed wasn't the first to reach the end.

As for speeding this up water channels and sprinklers work well once you have iron. Also try and do your mutation breeding with 10-10-10 stock as that will give you a resulting 5-5-5 seed to level up with.

For example: LVL 10-10-10 wheat + LVL 10-10-10 melon would give LVL 5-5-5 mystical white seed instead of the usual 1-1-1 you'd get from mutating with 1-1-1 seeds. Starting with 5-5-5 makes further leveling much faster.