When it comes to any game, there's a tendency for the other players to gang up on whomever is winning at the time. After all, it only makes sense. The best way to ensure one's own victory is to make sure that no one else is able to build up too much of a lead. This holds true in Mario Party Superstars as well, as it's easy to cooperate with the other two players on the board and in minigames to either frustrate or even outright sabotage the leading player. From there it's just a matter of keeping the heat on everyone else.
What if, however, you are the leader and you want to keep it that way? Well, it's actually possible to do so, but it requires a combination of gamesmanship, "boardsmanship" (more on that later), pure minigame skill and luck (this is still Mario Party after all). If one can combine these elements effectively, then not even a concerted effort from the other three players can prevent eventual victory.
Before even getting into the specific details for Mario Party Superstars, it's important to think about gamesmanship. Basically, one needs to think about how exactly to play effectively against the other players. Their tendencies, their strategies, the best moves to make in their position, their skill in minigames and even the items they have on hand all need to be taken into consideration when planning one's own moves each turn. Playing with randoms online makes this all easier since there's only rudimentary communication and therefore not many mind games to be played. One can just focus on what they think will be the best move for each player and then pay close attention to how well everyone does in the minigames.
Keep this info in mind and then plan moves/take risks based on that. For example, if one is close to a star and another player is close to Boo, then one needs to consider whether or not it's worth it to buy the star. If the other player can get to Boo next turn, but they need to win the next minigame in order to get enough coins, then it's important to think about how well that player has been doing in the games up to that point. If they haven't been winning, then it's probably safe to risk it and buy the star.
If they have been winning though, perhaps it's better not to buy and avoid feeding the star to them next turn. One could also consider whether or not it's worth it to throw the next minigame (if winning means that other player will get coins). While it would mean fewer coins in the short term, it also could prevent a star steal. It all depends on the situation. This last sort measure isn't as necessary in online play though since it's unlikely that the other three players will form an alliance.
What about among friends, though? What if everyone knows each other, can talk and are actively working together to keep you from winning? Well, that's where this gets interesting. The first thing to do is to attack the alliance. Appeal to the unfairness of it perhaps, or maybe take it turn by turn and try to help each member see the advantages of trying to win the next minigame instead of throwing it. After all, Mario Party tends to make people want to go for the win themselves, so appealing to that desire tends to work well. One just needs to do their best to avoid sounding desperate. Suppose this doesn't work though and the other three players are utterly determined to work together against you no matter what. Well then, it's time to get to work.
In this scenario, one can be certain that they will not win any 2v2 games or any 3v1 games wherein they are on the team of three (the other players will always throw these). They should also expect everyone to gang up on them in the head-to-head four-player games. It's not fair, but this can be the reality regardless. Fortunately, there are plenty of minigames in Mario Party Superstars in which the other players cannot gang-up and cannot deliberately lose in order to keep one player for winning.
One should do their best to practice these games in Mt. Minigames so that they can be almost guaranteed to win them whenever they come up; the same should also be done as the single player in the 3v1 games. In this way, one can still win a decent portion of the minigames played during a board game, perhaps even enough to secure the Minigame Star. If one isn't naturally good at the games, then this is going to take practice. Fortunately, there are some guides here to help one start winning quickly.
Next, one needs to consider boardsmanship; that is, using the board to one's own maximum advantage. This involves much of what was already discussed above, but with the added element of all three other players actively working to take you down. This means keeping careful track of everything: each player's position, coins, items, their comments and their skill levels. It's also important to consider the state of the board. For example, if the board is Space Land, the star is in the path of the Bowser Coin Beam and all three other players can affect the countdown, then one should probably consider not going for that particular star if the others can make the countdown hit zero while you're in its path. The same goes for Boo.
If they're all guaranteed to steal from you and at least one has (or can get) enough coins to take a star, then it's better to wait until the players who're able to steal are far away from Boo. If one buys a star before then, then it'll be the same as spending twenty coins just to give someone else a star. There are certain advantages to making someone spend fifty coins to steal, but it's only worth it if classic bonuses are turned on and them spending those coins would prevent them from gaining the Coin Star.
Speaking of bonus stars, it's also important to maximize one's chances to gain one or more of them at the game's conclusion. There are at least seven different bonus stars that tend to come up depending on the settings. Classic bonuses include: Minigame Star, Coin Star and Event Star, and are self-explanatory. The Minigame Star goes to the player who earned the most coins in minigames, the Coin Star is awarded to whoever held the most coins at any one time and the Event Star is taken by the one who landed on the most event spaces. For these, just work one winning as many games as possible, maximizing earnings in the special coin games and hoarding coins for as long as possible. Try also to stay near event spaces in the hopes of landing on them.
For new bonus stars, there's the Rich Star, the Marathon Star, the Shopper Star and the Slowpoke Star. If one plays things right, it's possible to actively work toward three of these at the same time. The strategy for the Rich Star is similar to the one for the Minigame and Coin Stars: win as many minigames as possible. One should also actively try to land on as many blue and lucky spaces as possible, however, in order to keep that coin count ticking upward.
It's also good to keep a Custom Dice Block handy in order to land on bank spaces when the coin bonus is above fifteen coins or so. Coins don't have to be hoarded in order to win the Rich Star, so don't worry about spending them as the situation calls for it. At the same time, make sure to buy an item at every opportunity so as to gain points towards the Shopper Star. It doesn't matter how expensive the item is, only that an item was purchased. If one is going for the Marathon Star, then items like Double and Triple Dice should be prioritized most of the time. The opposite is true for the Slowpoke Star, but chasing this bonus makes the game more difficult overall. Altogether, this should be enough to give one a decent chance of sweeping those all-important bonus stars.
Returning to the board for the moment, targeting the right players at the right times is also essential to maintaining one's victory condition. Stealing coins and especially stars via Boo, well-timed Cursed Dice or Chomp Calls, or proactively causing an event to prevent a star purchase or a star steal must all be things one carries in their back pocket. With no allies on the field, one must do their best to ensure that none of the other players can become a frontrunner for the others to rally behind. Just as teamwork is the other players' ally, chaos must be made into one's own.
If one is actively working towards the bonus stars and has luck on their side, then it should be possible to keep everyone else from developing a clear advantage. Of course, building up one's own star count is the most ideal, but that's not something to be depended upon in this kind of situation. It's going to take some real luck to do both this and keep one's stars from getting stolen over the course of the game.
One big mistake that many Mario Party players make is assuming that the its mostly a game of chance. Chance is certainly involved, and one is going to need at least a little good luck in order to secure a win against three players who are actively cooperating against you. But chance is not everything in any Mario Party game, much less Mario Party Superstars. Skill, gamesmanship and strategy are all much more important than chance, and they are what makes the difference between a Mario Party winner and a Mario Party loser.
Whether it's a fair free-for-all game or a 3v1 team-up situation, actively playing to win will always trump chance. It's not going to work every single time, but one would be surprised at how often they can win by just taking the game seriously. Want to win? Then forget about chance and work to maximize every possible advantage!