The key word in the trophy description is "guards," and although this is not entirely helpful for my trophy run I found that any enemy would do.
Fast travel to Albany (or New York, any place with a large amount of enemies constantly spawning) and activate only the Endurance cheat. From here on out, both the Morrigan and Shay will not recover any damage they receive. However, since you're on land, you only need to worry about Shay.
Now, be quick, careful, and try to do as many stealth attacks as possible as you, quite simply, rampage through the city area. Counter, break, and attack as needed. Unaware enemies are the easiest because you will one-hit kill them with your hidden blades. At 30 guards/enemies, the trophy will pop and you can de-activate the cheat.
Another way would be to attack the weaker ships in North Atlantic or River Valley (the former preferred due to the space it has), board them, and kill the enemies with either the Puckle Gun, your pistols, or any other ranged weapon so you don't need to enter the fray.
Either option should work but, for me, running around Albany with the cheat in took me only a minute to grab the trophy, even when I was reckless.