Ah, the utter confusion of a newbie. I've clearly been playing way too many games, to never end up asking questions like this.
Anyway, as Zen said, the "Green raindrop spell" is not a spell at all: It's actually a vfx that indicates the unit is poisoned and taking poison damage as a result. The green drops will play every round that the unit remains poisoned.
Hydras have an aura that poisons units that stand near them: You should generally avoid mixing hydras with normal troops that are not immune to poison, because your hydras will emit noxious fumes that poison everyone else. Think about it as being similar to standing next to somebody with really, really bad flatulence.
As a a general rule of thumb, hydras should not be mixed with normal, non-poison-immune troops (unless you REALLY, REALLY know what you're doing or REALLY, REALLY want them to die, which may very well be the case). Use them in combination with construct and undead summoned units, which will be immune to the poisonous fumes.
Those independents you fought are the company of Bogus the Troll. I have no idea what their backstory is, but they're a nasty, unfriendly bunch. When in doubt, just remember that excessive force is always an option, albeit not necessarily cheap or economical. Killing 70 ordinary schmucks singlehandedly is not that hard to do for a powerful, well-equipped unit. Try using either better troops, or a more unreasonable number of them, like 700. Or both: An unreasonable number of better troops.
Remember: As a newbie, you're in a unique position to use your newbie intuition: Do you, using your newbie intuition, feel absolutely confident you will win? If not, you need a bigger hammer.
[ March 17, 2004, 07:06: Message edited by: Norfleet ]