So, what can you do about this?
You can add a special item to each boss, for example (Boss item 1,2,3,4) . After the player kills each boss he can easily talk to an NPC and exchange the Boss Items (1,2,3,4) for the Quest items that are required in order to complete the Quest.
Then you can spawn the proper NPC and he/she can just finish the Quest.
Selling the source code L2-Scripts for the current revision of Sarviors and Zaken (Rev.26360) for further development or for the project, in 1-2 hands. The price is 300 $.
I spread the assembly for the test -> Download <---
Fixes can be viewed on their website.
All detailed information on contacts.