It's not cheating, but revives is essentially allowing the player to bring more than 6 pokemon in a fight by bringing back a knocked out pokemon, which isn't cheating as far as the game is concerned but is pretty unfair(a large reason the AI in any game never does it), plus it's just a bad habit to get into as you're not learning how to properly manage your team during battle when you should and might even use it as a crutch. The game only gets harder from here on and needs to have strong team options all around, not just cheap gimmicks. Hawlucha is a good pokemon not just for Samson, but for some tougher fights later on too which is why I recommended it. The guy above had a good strat with Sceptile too, one which I didn't even consider.
I'm not trying to just give him easy exploits to win(plus it can backfire anyway if Mismagius gets outsped and one-shotted somehow), I want him to learn the game in a way that makes him better at it too while really learning the different nuances and details of Reborn, which I feel are lost by revive spamming. Destiny Bond is already powerful enough alone, which is why it needs you to basically give up a pokemon to use it. By removing that drawback using revives you've essentially just broken half the game.
Sorry for being mean about it, I just really really dislike how abusable revives are which is a personal peeve of mine. If I had it my way you wouldn't be able to use them in battle at all but that's not how the game works so boohoo for me
But, the really important flaw with your suggestion, and worth pointing out most, is that he's locked within Agate and Route 2 ATM, meaning that he can't go back and get Misdreavus if he doesn't have it already. It's fine to give advice, but I think if someone sees a flaw in the advice given, they should be able to point it out. If Sceptile dude above told me that removing Sceptile is a bad move and posted his suggestione, I would've been alright with it.