Unlike most Pokémon games, evolution in Pokémon Legends: Arceus is done manually. You need to meet certain requirements and evolve them in your satchel. Turning your Hisuian Voltorb into an Electrode, which is an Electric/Grass type, is the same.
Luckily, evolving Voltorb is pretty straightforward: just use a Leaf Stone on it! Leaf Stones can be found in the Trading Post for 1,000 MPs, by breaking ores and battling the Pokémon that comes out (a Geodude will sometimes drop it, for example), or by treasure hunting using Ursaluna.
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Electrode is very fast, so make sure to raise the effort level (EL) of its Spd. Most of its move pool is also made out of special attacks, so bump the EL for that stat as well. It naturally learns Energy Ball and Thunderbolt, which are some of the most reliable same attack type bonus (STAB) moves in any Pokémon game.
Kantonian Electrode, which is a pure Electric-type, barely had any weaknesses. However, Hisuian Electrode’s partial Grass typing makes it vulnerable to four: Poison, Bug, Fire, and Ice. Fortunately, three of these (except Fire) are uncommon sights in the game. Plus, it’s not weak to Ground anymore and can learn more moves.
Hisuian Electrode is strong against Steel, Water, and Grass Pokémon—the latter two of which are commonly found in the game.
For more help with Pokémon Legends: Arceus, we at Pro Game Guides have you covered.