The magnet plant works, it makes enemy projectile slower until they are not moving anymore and than they are moving away from you, faster enemy projectile require more magnet plants.
For example, in the first world the pinkish tumor like thing with tentacles that fires projectiles that destroys hedges? 2 magnet plants will slows those shots and than makes them move away from you.
about 5-6 magnet plants makes things a bit easier, in regards to ranged enemies.
I do agree that the repelling effect could be stronger, not as strong as it was when picking it up once fully repelled any projectile, but it needs a buff, to be able to repel most projectiles after a distance of 3-4 hedges, if the magnet plant is picked up 3-4 times.