Jamming a technique is very effective in a fight. Using the traingle shape to defend - like crazy monkey boxing. Cut down the attacker - jam his attacks then attack the attack with your own strikes.
Spin crescent kick is better although not going to have a great % of scoring/striking with it. I personally just use crescent kicks for dynamic type stretching as other kicks are much more effectice. Round kick, side kick.
pure fitness footwork element. If you combine the footwork with throwing the shots as in fight then you'll be working on sitting down on your shots in between moving. Which is usefull when you face a fighter and need to counter punch with power to gain respect and prevent the attacker from walking through your shots.
To relax then mind must be empty of focus on nothing to do this you need to be unconciously throw techniques which needs lots of reps. Experience as in you've sparred lots of times and have the belief in your ability to relax.
If you just striking then the benefit a martial artist has is increased range due to the legs being allowed. Study Macheda's fight style and range of movement to beat the boxer. However depends on what type of boxer - boxer or fighter?
You can never have be too serious when sparring. I wonder if trying too hard is maybe whan the person meant. You have to empty your mind and focu on nothing so you can just react.