overkiLL is kiLLing your opp with high monster ATK with LittLe Life points , exampLe : you have rai-oh and endLess bLue-eyes and red-eyes bLack on the fieLd and your opp has 1000 LP Left .. you first attack with rai-oh then foLLowed up by red-eyes and endLess bLue-eyes .. that is an overkiLL .. to counter a chain Link 4 you need a counter trap card or something that can stop your opp for getting chain Link 4 .. chain Link 1 wiLL start if you use a speLL , trap , or a monster eff ..
List of cards that can kiLL in 1 hit :
EndLess Dragon w/ BLue-Eyes
Five-Headed Dragon
Chimeratech Fortress Dragon w/ Limiter RemovaL
Chimeratech OverLord Dragon w/ Limiter RemovaL
to counter the activation of an xyz monster's eff , you need an effect monster and a speLL card that can negate the effect of the xyz monster