Truncate database in mysql server by half?
there is no single query for this, but first what you need to analyze: 


are all tables have a similar size or not?
if any relations explicit or implicit?


often case when only a few tables take 80% of space, and in this case, you can copy all other as is and a half for biggest tables.

if foreign keys enabled - you must prepare a proper sequence of queries
if no foreign keys - you must think, do you strictly require data consistency or it just a test data and this moment not important