Cactus is one of the 20 cards added in Beta 6.6. Previous Versions can be found here.

This is a decent defensive card. It can kill any monster damaged by this one, no matter how much HP they have. Of course, this excludes cards that don't get damaged at all in a certain situation, such as Mad Dummy or Sans.

However, it has 0 ATK, meaning that it won't kill anyone unless you use cards like Snowdrake or Strength on it. And even when you have done that, people can avoid it because it has no Taunt so you may either need Protection or Glad Dummy to make it a target or use it as an offensive card.

So this card is best used against decks based on taunt, to get rid of them; it is suggested to use it with Bravery or in combination with Toriel; you can also make this a glass cannon if you prefer. To do this, you have to raise it's ATK to at least 1, which now also makes cards like Nice Cream Guy and Grillby an option. Secondly, use Madjick to swap its stats. This will give a 9/1 monster.

Countering Cactus is very easy. In it's base form, it instantly dies with Muffet's Pet or Madjick. And when the 3 card combo is used, you can easily solve that by Silencing Cactus with Scarf Mouse or Nacarat Jester.

If you use Lamp or Igloo you may get this, so it is recommended to keep an attack buffer in your deck, since Asriel can't do much for it.