// This compared player levels
Comparator:CompareLevel(left[PlayerData], right[PlayerData]) {
    // Returning negative means "left goes above". Positive means "left goes below"
    return right[Level] - left[Level];
}

// This is how you sort
Somewhere() {
    new sorted_by_level[MAX_PLAYERS];

    SortArrayUsingComparator(PlayerInfo, CompareLevel) => sorted_by_level;

    for (new i = 0; i < sizeof(PlayerInfo); i++) {
        new playerid = sorted_by_level[i];

        if (!IsPlayerConnected(playerid))
            continue;

        printf("player %d has level %d", playerid, PlayerInfo[playerid][Level]);
    }
}