Sorry for the long ranty reply I know I'm late but as someone who fell for this game before the shipwreck I'd feel cowardly for not replying on both the part of the Devs and the OP.
My Experience
As a new player I did expect him to be a pain after playing reborn and reju but even I couldn't beat him with my original team comp. I had Wartortle, Zigzagoon, pidgeotto, vivillion, numel and Venipede. I start with Numel to wipe out vulpix so I don't have to worry about drought more than once, usually I can get lucky and OHKO his next choice torracat but it usually outspeeds me and somehow takes over half my HP due to the boosted fire terrain despite Numel being fire/ground. Given the level cap I can't do much else but find a way to heal it which usually means sacrificing zigzagoon to his pignite which is now set up to outspeed most of my mons. I tend to end the fire field with gust on one of my two flyers and try to use viv for it since Pidgeotto is my plan to get rid of his fletch's gale wings later but I can't get past his growlithe no matter how I go about it after managing to wittle away his pignite. Wartortle can't out damage his morning sun even at night time when it heals less and nothing else besides my numel can seem to down it but doing so has basically ended in a trade each time since I already used my super potions and it feels dumb to "waste" a hyper on the first gym. Not to mention it's magby outspeeds my whole team from my experience. I tried to gamble with Venipede's rollout but he got revenge killed each time, pidgeotto seemingly can't kill ANY of his pokemon except pignite with Gust despite having a good EV/IV set-up considering this part of the game.
My Opinion
Am I saying that it's BS, no but I am saying that I don't think it's as easy as "lol get a water type bro" as some are suggesting. Mudkip would probably be the "best" starter pick for this gym but honestly I doubt that would cut it without proper counter set-up of rain dance AND water sport which will likely be martyr pokemon given the level cap at this point of the game. If you need to go that hard on the first gym it seems like a bad curve imo. Going to start a new save for now with the beach starter I was originally going to use and see how that goes but long story short, I don't think the curve for the first gym in any game regardless of the difficulty you're trying to sell should require specific move sets to win. Not because it's impossible but because it feels gross personally to have to team swap at each gym just so I can continue. For my two cents, as much as I've loved Reborn, Reju and especially Desolation I think it would be more enjoyable with either early access to terrain swap moves to counter gyms "fairly" or allow you to play a-la Ash and skip gyms until you actually have a team comp able to go against it. I liked the idea of level capping originally and I definitely prefer being able to "lose control" of my pokemon for a few levels to evolve it or teach it a necessary move before candying it back down but the way it's been implemented with essentially Smogon level Monoteams against players with Vanilla moves it just feels a bridge too far sometimes and Connor is one of those times. Julia was honestly a cakewalk with various options, Venam was somewhere in the middle but Connor just feels like a STAB Sucker Punch to my poor Ghastly soul. Thanks for your time and if devs read this late reply to an old thread I appreciate your time and work and I saw some reworks might be headed for early game that would affect this so obviously take this with a large pinch of salt. I love your game and I will persevere like the Dark Souls player I have become.