ALL hydra and ogre kills MUST be done while on a mission on a planet. For example if you do winters run (Archon Priest) as a nightfall strike, that level has 5 hydras, so, one would think you'd get 5 hydra kills... In that case you'd be wrong. Those count as nothing, along with if you get a hydra kill in a ROC or tiger strike.... It MUST be hydra kills only while going through Venus, selecting "Winters Run" and obtaining the 5 kills through the strike as run normal... Doesn't make sense, but that's the only way to obtain your hydra kills.