The requirements are:
You have well leveled Hammers
Your enemy doesn't go for a Grenadier Rush
It is also recommended:
Well leveled auxiliary unit (Assault, Heavy Assault, Fire Assault, Cyclone or Fortress)
Well leveled Assault Vehicle Factory
Your enemies rank is at most 1 rank higher than yours
No rank requirement besides having unlocked Hammers
The map is medium sized
Build your buildings in the following order:
2 Barracks
HQ2
Optional: Bunkers to deny enemy scouting
Optional: another production building for auxiliary units (Heavy Assault, Fire Assault, Cyclone, Fortress)
Do the early game normally (scout, get containers etc.) and start producing your Hammers and auxiliary unit(s) as soon as you have the required buildings for it (see Build Order section above). Meanwhile try to not get your base scouted and pick off as many enemy scouts on the map as possible and try to clear a path to your enemy's base.
Always remember: Resistance players can hide Riflemen in Forests which will alert your enemy of the incoming attack. Therefore it is advised to keep your Hammers in the back of your base (preferably not directly besides a forest) and only move them as soon as you attack
You should start to attack your enemies base depending on your rank after 5 to 9 minutes (5 in Legendary League and 9 in Steel League) to avoid giving the enemy time to build Hawks/Vertex which would easily crush your rush.
When you arrive at your enemies base it is recommended to order your Hammers with a single tap (they will attack while moving) to go to the back of the base as this makes it easier to avoid enemy defenses. Hammers also receive less damage while they move while also being one of the few units to barely loose any accuracy while moving. Meaning: Always keep moving!
Optional: If you brought auxiliary units you should order them to accompany the Hammers to the back of the base. You especially want your infantry to get away from those Sniper Towers in the front of the base.
You should now prioritize the following buildings:
1. Avia Factory (blue / red) or Airfield (blue / red) - choose the first one if your enemy doesn't have Hawks / Vertex yet, otherwise go for the Airfield (blue / red) to keep them from rebuilding their avia.
2. Assault Factories (blue / red) - skip if they aren't built in the back of the base
3. Headquarters (blue / red) - if you destroy it than you already succeeded in your rush (exception: Your enemy uses HQ Reconstruction boost).
4. Administrative Buildings
Note:
Switch to another Tactic if you see that your enemy is preparing a Grenadier Rush / Heavy Assault Rush or use Fire Assaults or Cyclones as auxiliary units against those.
Heavy Assaults as auxiliary units are especially well suited to defend Hammers from Dragonflies
There are no Resistance ground units which can hunt them. Only Riflemen, Coyote and Jaguars can keep up with Hammers of which Riflemen and Coyotes can barely damage them as they are anti-infantry Units. Jaguars in turn are inferior to Hammers while moving and their Siege Mode takes their mobility away which obviously would keep them from pursuing. Conclusion: Always use your speed advantage!
Walls (blue / red) are known to mess with the pathfinding of Hammers and can seriously hinder your rush.