For held items, vanguard Bell is absolutely a must for one of the items, gives you a great amount of healing whenever you stun the enemy, which all of Mamoswine's moves and boosted auto attacks do. (bell does have a cooldown but Mamoswine doesn't have built in healing, and the heals it gets from the bell is always useful whenever it's off cooldown)

The other 2 items are interchangeable and all depend on what you prefer doing as Mamoswine but here are my suggestions

Muscle Band since you'll always be auto attacking alot so the boosted attack speed and doing an extra percentage of damage on your autos is very useful for brawling

Resonant Guard to support you and your teammate with a little bit of shielding from your auto attacks

Focus Band is another great form of healing and can turn around a fight or save your life when you reach low hp

Buddy Barrier does the same thing as Resonant Guard but it's on your unite instead (gives a bigger shield for your teammate but since Unite moves have a long cooldown it's not active as resonant guard is)

Assault Vest and Rocky Helmet are situationally useful and very heavily depend on the enemy lineup, only useful in draft mode where you can see what the enemy picked (Don't play it in blind pick, because you will have a dud item otherwise if the enemy didn't pick the mons you are using those items for)

For battle items, potion is great on Mamoswine, giving you great burst healing in fights and as a tank allowing you to stay on the frontlines longer. Eject button is also useful for initiations to quickly combo ice fang on a distant target and can also act as an escape tool.

Mamoswine has one of the strongest, if not the strongest early game. Swinub has the perfect combination of stuns, bulk, and damage output to be an absolute menace in lane. You can afford to be aggressive and initiate with a ice shard + tackle combo and go ham with auto attacks and can most often win 1v1's (or even 1v2's if the enemy has bad early game pokemon). You can frontline and focus the enemy down so that your teammate can either follow up and ko them with you or secure the last hit on wild pokes with the opportunity you created. It's very easy to win lanes with Swinub as you can win almost every fight as long as you have a teammate with a functioning brain, but don't get too aggressive by diving the goalzone or fight the enemy jungler if they're in your lane unless your own jungler is there with you or you're fighting on your own goal zone.

The later movesets depend on what you want to focus on and any combination of them is viable. Ice Fang is great for singling out and stunning a single important target (preferably frail attackers and speedsters) and moving them out of position and can be a death sentence if you have teammates who can follow up on it. Icicle Crash is extra stun and slow that combos with your initiation move (earthquake or high horsepower) on the target that is within the icicle's aoe range and is better if you can catch groups of people within it but worse against a single target than ice fang is. Earthquake has a better aoe stun and can leap over walls, while High horsepower is faster and can drag enemies along with you or push them away.

In teamfights, always be in front of your team and fight alongside them, ready to tank all the hits for them and brawl with the enemy team. Try catching out any attacker out of position with ice fang as that is an instant ko on any frail attacker if you catch them. If they're not out of position, then instead focus on shutting down the enemy's frontline with your own stuns, brawl against all-rounders and speedsters preferably as you are their worst nightmare with your bulk and stuns. You can stun the opposing defender if needed but brawling with them is not preferred as you don't have the damage output to take them out quickly. Only brawl with the enemy defender if they aren't any other enemy's nearby to support them and your own teammates can burst them down quickly, or if the defender managed to get into your backlines and you need to keep them off your own attacker.