advice on cactus and corn please

Cactus overall can do some serious damage at long range, always be on a lookout against any threats like Scientists since some may plan to go after you for lacking the abilities to escape and have terrible hip fire accuracy.
The Potato Mines are very helpful when placed well since not only it can do tremendous damage it can alert you and everyone else if getting flanked. I recommend using Potato Mines over the nuggets.
Tallnuts are tricky to use, but they do help getting to high ground that is impossible for a Cactus to do so. (Additionally, it can block off Energy Balls during the last objective in Moon Base Z) I prefer Iron Maiden when it can protect you against stuff like ZPGs or Imp Punts. (You can still climb over one as well)

Now for the variants:

Fire Cactus has Fire damage that Scientists and Imps can't handle, but the penalties go to impact damage and reloading is slightly slow.

Electric Cactus has Electric damage good enough to take out grouped up targets and slightly faster fire rate, but it the same penalty as Fire Cactus.

Ice Cactus has Ice damage that freezes the target, but same penalty as Fire Cactus.

Future Cactus (my favorite) has 3 charging levels and very good accuracy at any range. Each charges increase your damage dramatically while the uncharged shots have a fast fire rate, 2nd for having twice more damage with 15 splash damage, and 3rd for 5 times more damage with the splash radius and damage twice effective. She doesn't have an effective penalty really, but the players will hate you if very good at aiming.

Camo Cactus is a heavy hitter with high magnification aim, your shots pierce through 1 target, and a ×2 critical hit multiplier. The penalties however are harsh for having the worst hip fire accuracy, low ammo pool, slow rate of fire, it affects the aim sensitivity greatly when zoomed, and very slow reloading time.

Bandit Cactus is more of a medium range class with Gatling Needles. She is not as superior as most variants to me.

Jade Cactus has increased health at the cost of her speed. Her shots isn't as powerful as any other Cactus, but it does have splash damage to clear off the zombies.

Zen Cactus is also a heavy hitter. Every ammo you spend increases the damage by 10, it also has splash damage. The orbs floating around does make you an easy target however.

Petrified Cactus is the same as Jade Cactus. The difference is that she shoots eggs that can be detonated at anytime before impact which does help against those in cover and multiple targets. She is tricky to play when you have to be good at knowing when to detonate.