Go on patrol on Mars, head to where the vex teleporter thingy is that takes you to the black garden area. A Hydra spawns there every 30 seconds or so if you wipe out all the ads in that area. Its a few hobgoblin and minotaurs. I sat on a rock with icebreaker and killed them all. Would do 15 minutes every now and then to slowly increase my numbers.
The only Hydra that won't count are any yellow health majors / ultras. Any red health bars all count towards your total. so there are a few strikes where you can get a couple.
Bro, I haven't patrolled in Mars in so long, I almost forgot the area you're talking about... I'll give that a shot.
Time to pull the Ice Breaker out of the Vault.
*random side note* - for me, getting an Ice Breaker to drop was so great... Still one of the all time Destiny moments for me in Year 1. Changed the entire game for me last year.