I can usually take hulks down pretty easy on foot, even if slowed down, unless I'm playing a melee character. Oddly, they give me the most trouble when in a vehicle. They're nearly as fast, and unless I can get enough speed for a one hit kill they get stuck in the vehicle, effectively pinning the vehicle as accelerating from 0 MPH does no damage. I imagine they're literally pinning the vehicle down or something, and then proceed to implode my skull. I've had that happen before being caught off-guard with a sufficiently large enough horde to anchor a vehicle.
More than once I've clipped a hulk along the driver's corner of my car at around 30 MPH, and it passes through the vehicle though not long enough to completely stop it. The hulk 1 or 2 hit kills me as it I'm running it over because the tile is adjacent.
Fungal spires are easy. I've killed one before noon of the first day. Just fire alone isn't enough, you want a shotgun or a high powered rifle. Blow the outer wall away when the fire can spread into the tile and the outer wall doesn't seem to grow back if you can get a raging fire burning in it's tile. I've never seen the fire do any real damage to the spire itself however.
Triffid groves I've NEVER killed even using the debug menu to cheat ultra skill and equipment. The vines are impossible, and even if you get into the last area the walls will trap you. Even with a jackhammer and explosives I always run out and get trapped before finding the heart. Queens are easier, just get a vehicle without windshields. Drive right up to the edge of the trees where you can just barely see the queen, blast away with a high powered rifle. Drive off before the triffids and vinebeasts catch up.