My strategy is in the comments

Give it a Modest Mint if its nature isn’t already Modest. Maximise the Special Attack and Special Defense EVs and use the rest on HP. Hyper train the HP, Special Attack, Special Defense, and Speed (Attack and Defense don’t matter because you have no physical moves and neither does Typhlosion, but I maxed my Defence anyway). Remember to change the Tera Type to Water.

If its Ability is Sap Sipper, use an Ability Capsule to change it to Hydration. Don’t use an Ability Patch like I did at first, because that will change it to Gooey and you will have to use another one to change it back.