I don't think you can filter on give me people who have gained 25 points this month. However, you could set up a surrogate where a second field is measuring partner engagement and ratchet it up that way. Say you have a field called Partner engagement with different tiers.
No engagement = 0
Engagement level 1 = 1-10
Engagement level 2 = 11 - 20
Engagement level 3 = 21 - 30
Engagement level 4 = 31 - 40
Engagement level 5 = 41 - 50
You get the idea can have smart campaigns move people from one level to the next.
You can report on change in this secondary field with constraints of change data value in previous 30 days or between dates, and if you want to go deeper you can use previous value, new value constraints.
The other way you could do it is also decay them at the end of each month so they are all on the same baseline but keep their engagement level in the secondary field if you don't need an exact number.
If you want to get an idea of what is happening first before you go into building a complex tracking system is to download the list at the beginning of the month, and at the end of the month and in excel do the math to see what levels make sense and build accordingly to see how behavior scores are changing and by how much.
I'm sure there are a bunch of different ways that you can solve this but this is the first one that popped into my head.
You could also use Score Was Changed IN PAST 1 month.
Overall, I'd like to better understand the goal here. Is this to check on leads owned by the Channel partner? How is this process setup with the Channel? Channel is one of the most complex things to setup across Marketo and systems and there is something missing in the question.