Treasure chests are random enemy spawns that occur (unaltered in the wild) about 1% of the time. Kill 10,000 of them.
First, a perspective of what this might look like. If you had unlimited dps and were knocking enemies out as fast as the game would allow you, you'd be seeing just under 2 enemies per second {(1 frame for death + 15 frames to respawn) x 2 = 32fps.] If it was truly guaranteed that 1 in 100 enemies were treasure chests, and you were insta-killing your targets, it'd take approximately 58 days of straight play time.
That's a bummer. Here's a couple things that may help speed that up:
After you're a ways into the game and you start ascending and cashing in your Hero Souls. There is a summon you can tag in named Dora, The Ancient of Discovery (/ The Explorer), you can buy to help raise the chance you'll see a treasure chest. Dora isn't cheap, but it's definitely worth it.
After you've started transcending as well, there is an outsider named Sen-Akhan that can help you see more treasure by boosting Dora's effect.
Here's some other things that I discovered:
• Turning off any game settings (ie: animations) does not seem to help the enemies die faster.
• Summoning Mimzee, Ancient of Riches (+to gold dropped out of treasure chests) I thought was helpful to accompanying leveling Dora. Made those treasure hits more exciting when they pay out more.
• Mixed bag of theories out there saying that treasure chests spawn gradually go down the further you go into the game. I thought it definitely felt that way, but cannot confirm. So if this is the last trophy to get, maybe stay low level.
• Autoclickers can be your friend and I thought were crucial at getting the ball rolling. If you're looking for something to throw your gems at in the beginning, I say go for it.
It was only after I had spent my vacation time to try to finish this trophy that I found a popular "exploit" that's out there that may help speed things up by cutting the respawn time to almost nothing. (~15kps!? NO WAY.) It involves having a certain mode on and holding a certain button. (Sorry, there's marathon runners in here and I can't spoil it for them. Even though you may stumble across it on accident.)
Happy sailing, friend!