Everything is pretty easy except for making the frog sprites. I'll leave that up to you.
Considerations: If you make the frog sprites small enough , they can be embedded in a net map as a shapes patch. Then you can also embed the specific frog cyborg physics to go along with it.
Changing the directions would require Lua, but would just require checking the projectile height vs floor height and randomizing the facing direction accordingly. Since this would either need to be in a plugin or hosted separate from the map in multiplayer, I'd suggest going ALL OUT on a new multiplayer gametype.
Why not make the hill the "core", players have to shoot frogs onto the core, which gives points. However, players can intercept the frogs with their body. Thus players have to decide whether they want to shoot for score but be defenceless, or deny players and then use the downtime to score their own points. Maybe players have a weapon that shoots frogs or grenades. Is he going to shoot a harmless frog, or is he going to shoot a damaging grenade? Oooh, the excitement! Obviously projectiles will have to move slower to allow dodging.