I'm assuming this is for Gen 7? I'm not overly familiar with monotype but I'll give my thoughts anyways!
For Electric, Rotom (especially Wash) is definitely a handy Pokémon to use as it avoids a Ground weakness overlap (unless Mold Breaker) and can act like a bulky pivot, Defogger, or even a breaker with Choice Specs. Since you are allowed one legendary, Zekrom can definitely be a terrifying Pokémon and it can act like a wincon with Hone Claws + Substitute. The final slot can be Tapu Koko thanks to Electric Surge and can have flexible movesets like a mixed set, special attacker, or dual screens to support the Zekrom sweep. However, I'm unsure if it counts as a legendary spot or legendary just means Uber.
For Ground, since Groudon is banned, Zygarde-Complete can be your legend for this team. It can run a Coil set and a Dragon Dance set and either of those can be a wincon. If you really want, the last two spots can just be Hippowdon and Excadrill; former sets the sand for the latter to outspeed and overwhelm teams thanks to Sand Rush. If you are concerned about a Water weakness then you can also consider Storm Drain Gastrodon to absorb it.
I unsure about Rock apart from doing a similar stat with the Ground team by adding Tyranitar + Lycanroc and perhaps add a Storm Drain Cradily to absorb Water attacks. However, Terrakion is a really good Pokémon to use instead of Lycanroc because it can double dance (Rock Polish + Swords Dance), run a Choice Band to melt teams, or run a Choice Scarf to revenge kill opponents. Actually, just use Terrakion, it's better than Lycanroc.
Finally Grass. First Pokémon that pops to mind is Ferrothorn as it set up hazards and is a good Pokémon in general due to many resistances. Virizion is a decent wincon because it can run either a Swords Dance set or a Calm Mind set. And then there's Tapu Bulu who can set up Grassy Terrain, break teams with a Choice Band set, Bulk Up, or even Swords Dance, as well as clean teams with Choice Scarf. Edit: How could I forget about Kartana? That can replace Virizion as it too can use a Swords Dance set and it's quite fast too!
Hope this helps and good luck!
