Use against players with high dribbling,agility and balance to make sure your defenders stay on their feet when defending against them.this will make it much harder for opponent to dribble past or win a free kick...
it Depends if he is an aggressive player ot not...If he is not aggressive you can live it on normal...You don't want him to play like a sissy...It is also useful to check his decisions...If he is aggressive but has good decisions you can also put him on normal...Each Player has his own PERSONALITY so choose accordingly...Also observe carefully how many fouls each ones makes...If you see someone makes too many fouls reduce his tackling instructions...If a player has a Sporting or Honest personality you know that is not a dirty player so put him on hard tackling...If someone is determined and you are chasing to equalize he will tear them apart...You also need to take into account the Referee personality..If he is lenient (merciful) Smash them all..If he is Harsh don't let anyone on hard except the most trusty players...I use this instruction every time one of my players get an yellow card to avoid receive a red one.