To add a little information for your understanding:

Lv 30 > 59, Trick attack adds your AGI value to the damage value of your weapon for a single hit, as long as there is another player directly between you and the monster that you are targeting.

Eg: YOU > PLD > < MOB where the arrow shows the direction that person or mob is facing.

With a 20 dmg dagger and 50 AGI in this case you do a single hit equivalent of hitting the mob with a 70 damage weapon.

Due to it not being a guaranteed crit, it's generally stacked with Sneak Attack. Since Sneak attack requires you to be stood behind the mob you need a setup like this:

YOU > MNK > MOB > < PLD

Lv 60+ Trick attack adds your AGI value to the damage value of your weapon for a single (critical) hit, as long as there is another player directly between you and the monster that you are targeting. Due to this being a guaranteed crit, it's no longer required to stack it with Sneak attack, so the first diagram works best. Also since at 60, you have access to a bit of haste gear (at least 12% from what I can think off the top of my head), mages all have access to haste spell, and BRDs have access to March, and you have your first tier of Triple Attack, and DW 2 as /nin, you might be able to WS every 30 seconds now, so you can stack each with a ws separately.