Hydra's in public events on venus do not count for Grimoire progress... as well I noticed that Iron Banner Clash also did not count for any grimoire progress to the clash card...
I'm not sure if this is broken or they just missed it in production but please fix.
-
Do the black garden story mission. Get to the part where you scan the two spires. Kill the hydra on the right hand side and then kill yourself. Rinse and repeat. I got around one hundred kill in one hour.