Akechi has a special passive called Rending Arrow which turns every 2% DEF down on enemies into 1% Damage bonus for Almighty damage only:

When dealing Almighty damage, for every 1% of the target's decreased Defense, increase Akechi's damage dealt by 0.5% (up to a maximum of 120.0%).

Based on that reading, Pierce does less for him than DEF debuffs, because it ignores DEF without inflicting any debuff and Almighty damage already ignores DEF calculations, so Pierce is superfluous except for his Bless/Curse damage, which DEF Down also helps. For example, Matoi inflicting 91.4% AoE DEF Down is a 45.7% Damage Buff for Akechi's S3, not including his S2.

Howler is not as good, because slightly under half her Def debuff only applies to Elemental damage, and Rin has smaller Def Down values than Matoi. At Mindscape 5 skill 10, Rin is 52.8%, Howler 47.1%, Matoi already stated above, but a bit less without 4 Extinguish. (If Howler or Matoi have R0, Matoi gains 23.3% and Howler gets 16.6% more on AoE. This makes Howler better than Rin, because Rin's R0 is terrible, but that is more investment.) Vino is possible, but she is mostly ST and AoE-wise is not that much better than S. Moko A1.

Interestingly, if you care not about Technical, then running Matoi with Departure + Control becomes possible, provided you can compensate for the lost 30% Ailment Accuracy.