Laying laminate in kitchen, need to lay under contiboard worksurface support

I'm replacing the kitchen floor. Have lifted current floor (line tiles - yuk) to reveal concrete floor.
I don't anticipate any problems, other than this one: we've extended the work surface since moving in and the end is propped up with conti board which sits on top of the current lino tiles. The conti board is also screwed to the main wall, so what I plan to do is to somehow trim about 5mm from the bottom of this conti board but it will need to be done in situ.
So what I need as a tool which is capable of cutting through conti board approx 12mm thick but at an angle of 90 degrees. I was thinking maybe of a circular cutting bit to fit into a power drill - do such things exist?
Last resort I suppose would be to use a hand saw, but that will be a bloody nightmare job.
Any recommendations gratefully received.

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