[Nero_3D]

You already got a kill variable, dont you? I will call it gPlayerKills.

pawn Код:

// OnPlayerDeath
    if(killerid != INVALID_PLAYER_ID) {
        new
            expGain,
            kills = ++gPlayerKills[killerid]
        ;
        if(kills == 1) { // for the first kill
            expGain += 10; // 10 - 20 exp
        }
        if((kills % 5) == 0) { // for every fifth kill he gets bonus exp
            expGain += kills; // 5 kills = 5 - 10 exp | 10 kills = 10 - 20 exp | 15 kills = 15 - 30 exp
        }
        if(reason < 16) { 
            expGain += 10; // melee kill = 10 - 20 exp
        } else {
            expGain += 5; // range kill = 5 - 10 exp
        }
        expGain += random(expGain); // random that he could get the double exp
    }

That were my toughts, add what you want