Custodes have many amazing strategems - maybe the most best for any current codex. Some of them cost several command points so you should always choose them wisely. Note that these tips assume you want to play Custodes competitively so you’ll want to bring another detachment and it’s a good idea to bring a Warlord with access to a trait or a relic that lets you regain some of your command points. For example I run a battalion of Ultramarines and pick the Warlord Trait that allows you to regain each spent command point on a roll of 5+.
One of the best strategems for Custodes is From Golden Light They Come which for one command point let’s you place one unit of Custodes in deep strike reserve. Note that Allarus Custodians (terminators) and the Shield Captain in Allarus terminator armor both have the ability From Golden Light which allows them to automatically setup in deep strike reserve without having to use any command points. I use this Strategem for my Wardens so I can deploy them in a strategic position such as behind enemy lines if possible or beside a flank to harass the enemy. Wardens have the ability Binding Oaths which helps them to ignore wounds making them more robust so they are well suited for this role.
I’ve found that versus opponents who also have placed several units in deep strike reserve it’s often better to go second so you can better react to their deep striking units (e.g. , Obliterators). This forces your opponent to place these units more cautiously since you can easily retaliate.
Another great strategem is Concussion Grenades which also costs one command point. This strategem allows Allarus Custodians (terminators) or the Shield Captain in Allarus terminator armor to fire their Balitus grenade launchers such that if they hit an enemy unit these units cannot fire overwatch and it reduces their ballistic and weapon skill by -1 for that turn. Due to being able to split fire now in eighth edition a squad of Allarus Custodians can target separate units, up to one for each terminator. I use a Culexus assassin and if you deep strike it in close proximity to the terminators the same turn then enemy units struck by the concussion grenades cannot hit the assassin, shooting or melee. Custodes are BS2 and each grenade launcher generates D3 attacks so you’ll more than likely hit each enemy unit you target. Also note that the grenades have 12" range so your terminators can shoot past the 9" bubble of separation required when deep striking next to enemy units.
The next strategem I want to cover is Vexilla Teleport Homer which allows you to place one unit of Allarus Custodians (terminators) or a Shield Captain in Allarus terminator armor wholly within 6" of a Vexilus Praetor and more than 3" from any enemy unit when deep striking. This strategem costs three commands but it can be a real game winner since it virtually guarantees a charge. Bait the enemy close to your Praetor then spring the trap. If your opponent is aware of this strategem then it makes them all that much more conservative with their movement phases.
Another awesome stratagem for Custodes is Shoulder the Mantle which also only costs one command point! If your Warlord is slain you can pick a Shield Captain to become your new Warlord and he automatically generates a new Warlord Trait (i.e., Superior Creation - 5++ Feel No Pain). This denies your opponent both Slay the Warlord and King Slayer... that’s huge and can also be another potential game winner. I always run two Shield Captains so I could use this strategem twice in one game - so dirty.
Victor of the Blood Games is a stratagem that costs two command points... select a character during deployment and it can re-roll one hit, wound or save each turn. Combining this with a relic for a 3++ invulnerable save and Superior Creation makes your Shield Captain neigh on invincible.
The last strategem I’m going to cover is Tanglefoot Grenades which costs one command point. Choose an enemy unit within 12" of an Adeptus Custodes infantry unit at the beginning of their movement or charge phase and roll a d6 - they must then subtract this value from their move or charge. Note this strategem does not affect enemy units with the keyword Fly. A solid tactic is to place a Custodes infantry unit in a crater or forest which subtracts another 2" from the targeted enemy unit’s charge... potentially reducing their charge by up to 8"... evil!
Conclusion
So there you have it. The strategems I’ve covered above are my go to choices and the majority of these only cost one command point. They can all be game winners too. Again my advice is use them wisely... don’t blow all your command points the first two turns because you’ll need them at key moments later in the game. There are many other rock solid strategems to choose from as well - like I said above and I’ll say it yet again choose them wisely and only when you really need them. Go forth and conquer with this grand arsenal at your disposal.