25 foot+ high bank back boundry to garden...

do you mean it rises vertically to 25 feet or that it slopes back until it's 25 feet high? In other words, is it climbable? If the slope is very steep, to the point of being vertical, you're in danger of it collapsing if you remove all the roots of the plants - that's what'll be holding it all up. And if it is vertical, you're going to need a landscape architect to give you advice

how about getting it heavy blocked over with a water faecture at the front as long as the bank.you have to put a pond liner behind the blocks.slope it back slightly.get some natural rock pebbles etc.put a pump in the feacture at the bottem and run a pipe up to the to resoviour excuse my spelling.this should also be very level so the water runs evenly along the length.you can shape wear the water goes bye placement of the stones,cement.have the bottem feacture raised so you can sit and admire and it would also hide any uplighting you mite want.you can make little planters here and there down the drop to and miss them with the water again with placement of the cement/stone etc.it would look brilliant i think when it greens up and solve your weed and bramble problem.you could do this with just a part of it or do more bigger planters and less or know water.im just biast as i love my pond/feactures lol.personaly id make a cement man hung and looking like a man in the old fashined dungeons if you know what i mean but thats just me.this will cost you but it will look brilliant for ages with a lot of benefits ie weed control for its lifetime,a brilliant in my opinion focal point,somewear to sit day and night and easy to maintaine.the downside is the cost but personaly i think you would get paid back in the long run in time and money.just an expensive idea of mine for a difficult problem.
ow and you get to keep fish to lol