Once a tournament is completed, to make the next tournament easier, dump all or most of your money, into pilot stats.
For some reason, pilot stats do not count towards the difficulty calculation, of the next tournament. You could potentially max your pilot stats, and still be fighting tournaments at the easiest difficulty level.
Whilst I don't know the exact values, a rule of thumb that I use, for the minimum difficulty setting, is as follows;
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North American Open - Robot should have all level 1's (no purchased upgrades)
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Katashi Challenge - Robot should not exceed the value of all level 2's (1 level of upgrade)
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War Invitational - Robot should not exceed the value of all level 3's (2 levels of upgrade)
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World Championship - Robot should not exceed the value of all level 4's (3 levels of upgrade)