Trouble looping through array

There are a few issues here:

• You're calling parent.mygame.getplayer(parent.mygame.getpturn()).getmonsterarray() on every iteration of the loop, which is at least a bit inefficient, and also makes the code harder to read.
• You should consider using a foreach loop unless you really need the value of i
• Your current array loop uses <= where it should use < - an array of length 5 has elements 0, 1, 2, 3 and 4, for example.
• You're showing the "winner" screen inside the loop for any player, not if all elements are less than 0.
• All these get methods are unconventionally named (e.g. getplayer should be GetPlayer), and some should probably be properties or indexers anyway
• This level of indirection violates the Law of Demeter very significantly. While I'm not dogmatic about this generally, I suspect you could make this a lot cleaner with a bit of work.

Solving just the array part, you could use:

bool anyHealthyMonsters = false;
// I've assumed the type of Monster here. You could use var, but you should understand
// what that means thoroughly first.
foreach (Monster monster in parent.mygame.getplayer(parent.mygame.getpturn())
                                         .getmonsterarray())
{
    if (monster.getMonHealth() > 0)
    {
        anyHealthyMonsters = true;
        break;
    }
}

if (!anyHealthyMonsters)
{
    Winner winners = new Winner();
    winners.Show();
    this.Hide();
}

A cleaner solution would be to use LINQ though:

if (parent.mygame.getplayer(parent.mygame.getpturn()).getmonsterarray()
        .All(monster => monster.getMonHealth() <= 0)
{
    Winner winners = new Winner();
    winners.Show();
    this.Hide();
}

However, if you're relatively new to C#, I would suggest concentrating on improving the design before you worry too much about this. I'd expect the final result to look something like:

if (parent.CurrentGame.CurrentPlayer.HasDefeatedAllMonsters())
{
    ...
}

where HasDefeatedAllMonsters() would be a method in your Player class, which would be something like:

return monsters.All(monster => monster.Health <= 0); 

(Even the parent.CurrentGame.CurrentPlayer part makes me somewhat nervous, but it's hard to mentally redesign a whole app without more context. I strongly suspect that you shouldn't need to ask your "parent" (whatever that is) for the game - this class should know about it directly.)