When a Peashooter uses his Hyper ability, he will not only run incredibly fast, but can also jump extremely high.
Strategies
With
This ability has many uses. It allows the player to get to unreachable spots of all classes except the Foot Soldier (and sometimes Scientist). Therefore, the player can use Pea Gatling to defend his or her point in Garden Ops or Gardens & Graveyards, or just get a vantage point for any other game mode. It is also good for retreating as the Peashooter runs very fast.
Against
There's also not really a counter to have against Hyper. Just do your best to vanquish the opponent regardless of their ability, and hope for the best. Increased speed, jump height, and the user already has the smallest plant hitbox in Garden Warfare, make Peashooters in Hyper a pain to land your shots at.
You can use movement-inhibiting abilities such as Sonic Grenade to stun the Peashooter, making him easier to kill. In Garden Warfare 2, you can use the Gravity Grenade to catch the Peashooter and kill him. (Remember, plants in Gravity Grenade can still shoot back.) Or maybe Biotic Pull if you are playing as the Z7 Imp. You could also try to run and wait for the Peashooter's Hyper to expire, making him much easier to shoot at.