My tips are as follow:
1. Headshots only the front / medium row Ganados.
2. Body shots for moving Ganados.
3. Try headshots female Ganados as they yield 200 points instead of 100 points.
4. Try consecutive shots as many as possible, even if it means firing only body shots, as the Salazar target can compensate for the loss of headshot points.
5. If there is explosive-holding Ganado among others, try to shoot a few those around before firing at the explosive Ganado. The points obtained this way is higher than firing straight at the explosive Ganado as all other Ganados will go down without giving any point.
6. Practice. In my 1st playthrough I sometimes needed as many as 20 retries to memorize the patterns and where to stand.
All my played games are exactly 1000G! I'm a 1000G completionist.
I hate DLC achievements, since they always ruin my 100% 1000G record.
10000G for 10 games on 7/12/2010
20000G for 20 games on 9/8/2011
30000G for 30 games on 6/5/2012
40000G for 40 games on 24/9/2012
50000G for 50 games on 19/5/2013