Walking Wake @ Choice Specs
Tera Type: Steel
EVs: 12 HP / 244 SpA / 252 Spe
IVs: 0 Atk
- Hydro Steam
- Draco Meteor
- Tera Blast
With this EV spread + Timid Nature, Walking Wake's Speed stat will be 1 point higher than it's Sp. Attack, (@ lv. 100 and lv. 50), which is relevant as when Protosynthesis activates, you'll receive a ×1.5 boost to it's Speed (same as Choice Scarf), rather than a ×1.3 boost to Sp. Attack.
As for the Item, I'd recommend Choice Specs, giving a ×1.5 boost to Sp. Attack (although it does it's locking to one move thing). If you don't want to use this set in the Sun, you could run the Booster Energy to still get that ×1.5 boost to Speed with Protosynthesis. Other options include: Choice Scarf, (if I can math, a ×2.25 boost to Speed when Protosynthesis is active) Life Orb, (×1.3 boost on attacking moves at the cost of HP) or for a more bulky item, Assault Vest (×1.5 Sp. Defense, can use Status moves)
As for the moveset, Hydro Steam is a given, as it's Water STAB that gets boosted by ×1.5 in the Sun (Instead of the usual 50% decrease Water moves get in sun). Draco Meteor is strong Dragon STAB, that has a manageable draw back (Lowers Sp.A by 2 Stages after it hits). Flamethrower for coverage that also gets boosted in the Sun, and finally, Tera Blast (Normal/Steel) To hit threatening Fairy types for Super Effective damage.
As for the Tera type, Tera Steel not only for the offense it gives with Tera Blast, butbecause it comes with a number of resists, namely Fairy and Dragon, which are Water/Dragon's only weaknesses. While being weak to Fighting, Ground and Fire, you quad resist fire before Tera, and Fire and Ground types are hit for Super Effective damage with Water moves, making it hard to get rid of.
Finally, I recommend having teammates like Koriadon or Torkoal, as they can easily set up the Sun and both have other utilities that can help the team.