You won’t see Cactuars out in the Archylte Steppe until you take a quick trip to the mountainous areas of the Eastern Tors. And to do this, you’ll need to complete Mission 14 - Defender of the Flock.
Hop on a Chocobo and travel to the Northern Highplain. Look for a large Adamanchelid standing in front of a Treasure Sphere and then look for an illuminated yellow circle on the ground to the left of it.
Jump up the ledge and then jump across to the other side of the cliff. Finally, walk towards the middle of this elevated area to kick off a short cutscene involving Sazh and a Cactuar. It’s at this point that you will encounter Cactuars throughout the Archylte Steppe.
Cactuars will run away if you approach them out in the field but they are relatively easy to corner and engage. They will run sideways to the edge of their patrol location and then allow you to reach them.
Cactuars are not difficult to defeat. They have 777,777 HP though so it can take quite a while to bring them down, especially with their Chain Gauge being at 777%, making it nearly impossible to Stagger them.
Cactuars will occasionally use a move called “Cactus Dance” which, if you let it complete the dance, will cause it to grow into a more difficult Giant Cactuar. They are relatively easy to defeat though and don’t require any complex strategies. Just be sure to keep your party members healed through the incoming damage.
They have a chance to rarely drop a 
Cactuar Doll.
The largest Cactuar you will have to face off against is Gigantuar, who is the mark of Mission 54 - The Bigger They Are…
