On defence, i always struggle to tackle the comp on the gainline. What i have started doing is getting a good defender (like one of your flankers but depends on the team, and position them a few metres behind the ruck. They can then follow the play and tackle players easier as they come at them from the side.

The other tip is not to sprint too much on defence, as you fly past the attacker if you don't get it perfect leaving a nice big hole.