Why are you limiting yourself to only Gen 1 pokemon in the first place??
heatran should scare it. Not gen one and can't get it yet but mawile won't like it at all. Anyways...
1. Fast destiny bond ghosts (your gengar maybe?) Do not let them sub if they have it and force them to kill your target to switch to the ghost. Either you lose your gengar to kill it or you force it to swtich out and get a free hit on thier incoming pokemon (probably a dark type to resist so dazzling gleam might be a good idea to carry) If it already has SD boosts it might try to baton pass them to something faster than gengar to counter bond however but most probably don't carry baton pass. Since gengar has levitate it will also negate sticky webs, which is useful. With a non broken focus sash you also could hit it hard at least once to weaken it before sucker punch killed you. You could hit it once and then switch out to something to finish it off predicting the incoming sucker punch as well to give you initiative. However entry hazards are going to force you to destiny bond if you want to take it out along with your gengar. They might just switch into something else however to force out the gengar then switch right back to mawhile predicting your switch out. They could also predict you to switch out your gengar and use a stab move to kill/cripple your switch in pokemon. Mind games!!!!
2. bulky/fast fire type (arcanine/charzard for gen 1)
Charzard y on free switch in assuming it isn't allowed to get sword dances up for super sucker punches (perhaps run substitute!). I'm not sure if they will like switching into a resisted faire/steal type move but if predicted a normal/rock type will wreck them on a non-free switch in.
3. Status it. Chloro venasaur can outspeed to sleep it then leech seed/solar beam/growth on it or switch out, but the sleep could miss and without sun it can probably outspeed venasaur (since you often don't run speed evs on mawhile or chlorovenasaur). You might be able to switch in to a resisted play rough but you'll lose a turn of sun. Mega venasaur might give mawhile some trouble and might be able to tank a non-boosted hit to leech seed/sludge bomb it. Willowisp from arcanine, moltress, or MAYBE weezing will cripple it. Defensive weezing might have trouble with taking an iron head from mawile though as it can't outspeed and of course willowisp is only 75% accurate.
4. Fast and/or somewhat bulky ground type like glyscor or ground type movez. There is not many options for gen one. Mega Kangaskahn can out speed it or simply live a sucker punch and hit it with an earthquake. Other than that I believe only mega aerodactyl from gen one can truly threaten it with earthquake but niether are bulky enough to reliably switch into a hit and an iron head will ohko aerodactyl if EQ doesn't kill. I would hardly say these counter it as neither wants to take a hit from it but they can at least hit it back hard. Bulky steelix might be able to force it out and can probably get up rocks as well as dragon tail if neccessary but steelix isn't that amazing overall and they will probably just get switch initiative on it.
(mega)Garchomp, (mega)blaziken, mega houndoom (nonmega has terrible defense even though it resists sucker punch), infernape, arcanine (intimidate), heatran, mega charzard (sadly x has better defense for the sucker punch but without boosts you should be able to take one on y), ferrothorn, escadrill (if no boosts), gliscor, steelix and aegislash are all decent or good counters.
If you have to use gen 1 pokemon then use the arcanine or hope that destiny bond gengar or statised sleep/burn will work out for you. I imagine a physically bulky arcanine with intimidate can take a hit or two. Charzard can counter it but it will take heavy dmg in as y and likely die if it's a boosted sucker punch. If you run substitute on the charzard y then you might be able to juke the sucker punch and either get your charzard behind a sub to take the next sucker punch OR get really lucky and have your charzard behind a sub when they switch out in fear of the incoming OHKO fire move. They could predict that and baton pass a boost they have, sub up themselves, or use a move besides sucker punch to break the subs (then perhaps predict an attack and sucker punch) but that is pretty risky. They might plan to sack the mawile without going for sucker punch then get lucky and finally sucker punch when you go to attack but my guess is that they will go for suckerpunch or switch out when faced with a charzard. Defensive arcanine with intimidate is likely the best switch in you can get in gen 1 and if you are lucky you can even avoid dmg by "predicting" a sucker punch. Moltres might also be able to scare it out if you don't mind using it. If mawile lives a hit from these with a small margin of life and will kill you with a sucker punch then definitely switch to something with priority or that also resists it's stab moves cause it's either going to sucker punch or it will predict your switch out and go for either iron head or play rough. These are just suggestion but I hope they help. Defensive heatran, aegislash, or rocky helmet ferrothorn would give it a lot of issues. If you count steelix as gen one then he might give it a little trouble but steelix is unreliable, slow, and specially frail. Edit: Is pysically bulky eviolite magneton an option to trap it? A power-up punch, sucker punch, or resisted STAB will probably still really hurt a magneton so I guess it's a bad idea.
I'm also thinking Hitmonlee, as it's one of the only 'mons in Gen I that resists Dark, and would would able to outspeed Mawile, but idk if you could OHKO before the Play Rough hits.
You'll be that unlucky guy that runs into a mawile with speed investment! At 90 base speed compared to hitmanlees 87 it will outspeed but the likely hood of jolly max speed mawile is rather low.
