Blizzard investigated reports about missing Ashava trophies and clarified you must have killed Ashava during the May Server Slam at Level 20 to get it.
Killing world boss Ashava during the March betas does not grant the trophy and being below 20 during the Server Slam and killing Ashava also does not grant the trophy.
The trophy was exclusive to Level 20 kills of Ashava during the May Server Slam.
Ashava Trophy
We’ve checked a few reports of this coming in and want to remind everyone that you must have killed Ashava during Server Slam at level 20. Killing Ashava during the March betas does not grant this trophy.
Being below 20 during the May test and killing Ashava does not grant the trophy. Killing Ashava in March at any level including 20+ does not grant the trophy.
This was a trophy exclusive to Level 20 kills of Ashava during the May Server Slam