What to do under pressure?
 

Attack? Defend? Grind? Tank? Drop shot? Approach?

Some things that work for me are at crunch time:
Always think of my technique
Serve and volley (even on 2nd serve)
Never go for a winner
ALWAYS lob instead of passing shot (within reason)

4.5 tournament player

Thinking of your technique forces you to slow down and play point by point. That's usually a good response to pressure. Just don't fall into the trap of guiding the ball.

The rest may depend on the opponent and what's working for them. But under pressure I think you should do what you do best. If that's not working, lobs, change of pace, occasional drop shot may do the trick.

If you're getting creamed, change something.