By the time you read this, Groudon will have left us with Armoured Mewtwo replacing it in tier 5 raid battles. Groudon still the best Ground type around so I imagine a lot of trainers tried to get as many they could, even if they had already caught a Shiny Groudon. It’s a tough fight, needing at least 5 people to safely take it down.
What I found to be incredibly annoying was whenever the Groudon raid boss used Solar Beam which would knock out my Kyogres in one hit. It didn’t matter what level they were, they would just go down before I had a chance to fire off even one Hydro Pump. Suffice to say the rest of my team didn’t last very long until I realised I could use Grass types since they resisted Solar Beam. Then I realised I could use Alolan Exeggutor because it double resists Solar Beam. Not only that, but Alolan Exeggutor resists all of Groudon’s charge moves! Roserade does more damage but it’ll faint in one hit if Groudon knows Fire Blast.
Alolan Exeggutor is Grass/Dragon and that means it doubly resists Grass moves, takes neutral damage from Fire moves and has a single resistance to Ground moves. This covers all of Groudon’s possible charge moves. Being a Dragon type, Alolan Exeggutor is weak to Dragon moves so there’s a 50% chance you’ll be taking more damage from Groudon’s fast moves. Don’t worry it won’t hurt too much if your Exeggutor’s level is high enough.
For moves you’ll want to stick with Bullet Seed and Solar Beam, they’re both Grass moves and you’ll be dealing STAB super effective damage. These are the best possible moves in terms of Damage Per Second (DPS) after I used Gamepress’ DPS spreadsheet to check how Alolan Exeggutor fares against a Groudon using Mud Shot and Solar Beam. In such a scenario, Alolan Exeggutor is the second best counter, losing out to Venusaur using Vine Whip and Frenzy Plant. Alolan Exeggutor isn’t too difficult to get while Venusaur with Frenzy Plant isn’t available in the wild at the moment. Alolan Exeggutor isn’t the best overall counter to Groudon, I believe that honour still goes to Kyogre in any situation where Groudon isn’t using Solar Beam.
So if you think Groudon isn’t using Solar Beam, go with Kyogre or the myriad of water types that do more damage than Alolan Exeggutor. Stick with those, but if Groudon is using Solar Beam, use Grass types such as Alolan Exeggutor.