1st Opening in 15 Years

Bought a house with a 40 year old inground gunnite pool - 22x45. 12' at its deepest end and 4 ' at its shallow end. For the last 15 years it was being used as a pond with fish and frogs. There is a lot of sludge at the bottom of the pool from pine trees and animals. What is the best way to get it out? I have read not to drain the pool but to filter it out slowly by first putting a lot of chemicals to kill everything the filter it out and keep cleaning the filter. Others have said to drain the water to as close to the end as possible then fill buckets up with the sludge and dispose. What is the best way to get the gunk out?