My suggestion for builds is not to build for the sake of building, but rather have a target mode that you want to do well in.
Chrom with Sealed Falchion is likely to gravitate towards EP (block follow up is good for that), and unfortunately there is little to no merits to only counter melee units. Enemy phase playstyle is mainly used in Aether Raids Offense, so you should probably look at preparing him specifically for that mode. Good skills to consider includes Spurn, Null Follow Up, and most definitely Distant Counter. Arena isn’t probably the best of idea due to his subpar scoring potential.
Erk is a little more flexible. Be it in PvE or Aether Raids Offense. In PvE, you can do no wrong with the typical AoE Spiral build. (Life and Death, Special Spiral with Heavy Blade). For Aether Raids, everything looks about all right except the weapon, in which you should look at predominantly enemy phase options. Red tome in this respect had it rough, as your only competitive option is Unity Blooms from a 5 star Henriette. Otherwise stick to Serpent or… Conch I guess.