In short version:
There are 2 ways to do this - easier and harder.
1) Easier:
Just make simple query action that you will attach for this chosen monster under its action task.
Query you will prepare composed just from 1080 and 1081 types of action for example:
(119, 161, 160, 1080, 2700, 'isexit'),
(160, 161, 161, 1080, 2700, 'new'),
(161, 190, 190, 1081, 2700, 'completenum += 1'),
then just checking amont of gathered completenums for this 2700(thats random number - example just) data:
(190, 200, 2001, 1081, 2700, 'completenum >= 100'),
so starting from 200 action make query you wish to be processed by monster after killing 100 of them so for example simple message like this:
(200, 0, 0, 126, 0, 'You just killed 100 monsters'),
and as 201 - meaning not yet completed:
(201, 0, 0, 126, 0, '1 more killed but still not enough..'),
Thats pretty all of logic under this simple example
2) Harder
You can(but as well proceed with 2nd idea without 1st!) attach first idea with additional visible part but this will require you to work on existing quest book or to make own one.
So you will need to work out the 500001 ID of book for this quest (it will be storing and showing on screen killed monsters amount)
and proper actions connected with killed monsters IDs
example action for this book:
(61470, 61471, 61473, 0528, 500006, '0 0 0 0 0 0 0 0 0 51001 0 100'),
Sorry for all mistakes and misunderstandings here. I wrote this in ultra speed in free momment from memory mostly

I wont make for you ready quest but with this + little tries I am sure you will be ale to prepare something unique
P.S. look out for this book quest part as its not always storing proeprly killed monster there (I dont know why but can happpen

)
Cheers