To perform a downthrust attack, jump while holding attack and down. You can hold the attack button and you will bounce from enemy to enemy. There are plenty of places throughout the game where there are enough enemies grouped together to get this trophy. I got it done in the very first dungeon but some say that the Moongate section is the easiest place to do it, with the yellow slimes. Just keep an eye out for opportunities and you'll have it done no problem. Very easy to do.