My daughter had a large space of garden thats been neglected for years, its all overgrown, uneven, bricks and weeds. Shape of a rockery 'steep incline' whats the best thing to kill it all off before wecover it in tree bark

Probably Round up or similar, but it does really depend on what's growing there. Thick black plastic pegged down over it all and left for a year will do the trick too.

a few applications of round up may be require if there are brambles and perennial weeds there. Brambles will lift up the membrane even if weedkilled and chopped, they may grow back