I have run into this before.
My solution was to get some 4x4's or something similar. With them laying on the deck, slide them to the boulders till they were like chocks. Due to the odd shape of the boulders they were not parallel but thats fine, cut some other 4x4's so they are the right...
I've used one of the old school hand pumps (mine happens to be from the 50's) and installed a filter on the outlet side of it. Snowing like crazy now so I don't have a picture but pretty much the same as the picture below. Easy, no power needed, fairly fast. Uncomplicated.