Plant Contra, also known as the Big Benny Plant, was a plant/cyborg hybrid that was one of the "Four Hell Warriors of Neo Contra", renegade Contra that were the elite members of the Neo Contra organization. He uses two forms during the boss level.
Mutant Benny Plant - Part A (budding form)
Arguably, one of the easiest bosses in the game. He begins closed up and throws seeds at you, which become little plant things that walk around. He will then lunge at the spawns, killing them (and you if you do not move out of the way). Occasionally he will slap the ground with two vines, creating a large shockwave.
The spawns can be killed as soon as they hit the ground, and are slow moving, so they pose an almost minimal threat. The only problem is when the boss lunges at the spawns. Simply avoid positioning yourself between the boss and spawn. The shock- waves are harder to deal with, but still very easy. The waves are created to nearby the boss, so stay along the edge of the room.
His entire body can be damaged, so simply begin firing as soon as the fight starts. Move out of the way to avoid the lunge and back into the middle to avoid the shock waves. In all likelihood you will be able to destroy this portion of the boss before it uses the shockwaves at all. Heavy weapons are the best choice, and if you use the Katana or the Flaming Katana chances are this phase will end before it can even launch the first spawn.
Mutant Benny Plant - Part B (blooming form)
After destroying the base, the flowery part will open up and the boss will become much more unpleasant. This time it will throw several seed pods on the ground which will all burst into a single bullet at the same time. The bullets are the standard ones that humanoid enemies fire, and the pods target your location when they burst. The bullets lack a homing ability, but there are quite a few on screen at once. To make things worse, two spikes are moving around the boss in circular motions, limiting your mobility and forcing you to constantly keep moving.
The bullets are easy enough to avoid, and you can destroy the pods before they burst. However, this takes time and you will most likely only get one or two pods before they fire. The real threat is the spikes moving around the floor, which move increasingly faster as you damage the boss. Keep focused on your character, not the boss and move before you get hit by anything.
As was the case with Guerilla Contra, the boss's weak point is the flowery part, which can only be hit with lock on weapons. Do not pay any attention to the boss itself; instead just hold down the lock-on button and focus on avoiding things on the ground. Once you lock all of your beams release them and start the lock-on for the next volley. While not a particularly hard boss, it will take quite a few volleys to kill it, so be careful. Like with Guerilla, the Weapon Set D is probably the one that will make things ridiculously simple.