[Misiur]

I don't think you should do this in OPU callback, I'd recommend using timer (5-10 seconds intervals). Now, in separate function:

pawn Код:

forward CheckRank(playerid);
public CheckRank(playerid) {
    new previousRank = pRank[playerid];
    switch(pInfo[playerid][Kills]) {
        case 100..300: pRank[playerid] = Master;
        default: {
            pRank[playerid] = Kid;
        }
    }
    if(pRank[playerid] != previousRank) {
        SendClientMessage(playerid, -1, "Hey, you've got promoted! Congratulations");
    }
}

"Default" will cover for both 0-99 and some random values (bigger than 300 in your case)