Ivies are very slow to move, and can be fought at range with a handgun in most of the games they appear in. In a close range fight however, they are weak against fire and will die quickly when attacked with a Flamethrower or Flame Rounds from a Grenade Launcher.
Ivies display a unique weak spot, gunfire to the center of the opened flower will kill it instantly. It's actually possible to kill an Ivy with two handgun shots. The first will cause it to rear back, and then open its flower-head for a moment, and a second into the weak spot will kill it immediately. This only works in Umbrella Chronicles due to its aiming system