The other answer (the accepted answer when first writing this) is wrong or incomplete on some important points.
The Ephemeral Cover Exploit is useless against angels:
The demon can craft a Cover out of spirit-stuff, creating a mask that resembles a ghost or a spirit.
He can harvest Corpus using this Exploit; as such, Ephemeral Cover also functions as an attack against ghosts and spirits.
A demon with a Cover like that can enter Twilight, but a ghost or a spirit in Twilight still can't touch an angel in Twilight, as the question above already mentions. And you can't use it to create a Cover of any other type of ephemeral entity, let alone to impersonate an angel, so a demon is never going to be able to attack them in Twilight by becoming 'of the same type' that way. (They could always submit to the machine for 'redemption,' of course...)
Gadgets are less versatile than Embeds and Exploits, not more, and while they could "bend the rules" with near-field effects that your Storyteller might possibly approve, there's by default no Gadget that can cut angels in Twilight, let alone all ephemeral beings indiscriminately.
Gadgets offset portability and reliability with inflexible single-function usage that represents only a portion of the Embed’s or Exploit’s potential.
The sample Exploited Gadget listed in the book fits the default rules for Gadgets, it's a strict subset of functionality: it only hurts ghosts, not ghosts and spirits, and it's highly dubious whether you'd be allowed to create an equivalent that would cut angels, because the original Exploit can't touch them.
A straight razor that hums audibly and spews a thin ribbon of fog is Installed with Ephemeral Cover (p. 164) and is capable of cutting ghosts.
Aetheric Resonance costs one Aether to activate it for every scene you'd wish to check for the presence of angels in Twilight or the use of their Numina, so to be on the lookout you may want to buy the one-dot Merit Resonance Sensitive (Prerequisites: Wits ••••, Flowers of Hell p. 129) to allow the sense to be constantly active at no cost of valuable Aether. (The truly paranoid might buy dots in Resonance Aware to greatly extend the range of the demon's aetheric resonance sense.)
Finally, the Bolthole Merit's "No Twilight" option is useless here, because boltholes are already even more useful than that out of the box. Angels are completely unable to enter boltholes in the first place:
All boltholes have the following benefits:
Wards: The bolthole is warded against angels (see Warding, p. 353).
As explained on p. 353, which are the same rules on ephemeral beings as in The God-Machine Chronicle rules update and the Chronicles of Darkness core book:
the entity described in the ritual treats attempting to move into or out of the warded area as though it were against its ban.
All ephemeral beings suffer from a mystical compulsion known as the ban, a behavior that the entity must or must not perform under certain conditions.
Now some potentially useful Exploits when dealing with angels in Twilight are:
- Reality Enforcement, which robs all nearby demons, angels, and stigmatics alike of their powers (it doesn't specify how long this effect lasts). Angels become unable to use Numina and ones in Twilight are forced to Materialize, at which point you can just shoot them with guns.
- The Word, which can give single-word commands to a target, which can inflict lethal damage or force them to e.g. 'flee' for a number of hours. It doesn't say the commanded target has to be human, solid, or even visible to the demon at all, so even invisible angels can be targeted if you know they're there. (Perhaps you could just command them to 'materialize.')
- Show of Power (Flowers of Hell, p. 114) is the wild card: it can mimic the Merits or powers of a non-demon if the demon has seen that power used before and if it doesn't require an extended action to perform, so if any other type has a way of hurting or repelling angels in Twilight, they can use that. (It says 'another type of supernatural being,' so by RAW you can't use it to mimic mortal powers like warding and binding, though that would be a fairly logical extension/houserule.)
Also, there's one more (general, non demon-specific) weakness angels share with spirits: Essence Bleed. An angel in Twilight roaming freely can't stay that way forever.
• Ephemeral beings outside of a suitable Condition bleed one point of Essence per hour. The Influence and Manifestation Conditions starting on p. 346 state whether they protect from Essence bleed for different types of ephemeral being. Entities that run out of Essence due to bleed suffer a single point of lethal damage and enter hibernation.
To avoid this it would either have to become Materialized, and vulnerable to physical attacks, or to inhabit a target's body with the Possess Manifestation, which renders them unable to use any of their powers (Numina or Influences). Outside of those Manifestation Conditions, their mission is on a time limit before they have to resort to those two options or retreat to refuel itself by returning to Infrastructure or calling on a local cult's sacrifices. It depends on how strong it is: Rank 1 entities have a Maximum Essence of 10, Rank 4 entities have a maximum of 25, Rank 5 can hold a whopping 50 Essence.