There are number of methods but its selection depends on the properties of reactant whether it multifunctional or unifuctional you may choose enzymatic amidation or silylation followed by condensation at extreme negative temperature what we always do.

I think that you need to be a little more specific!

I am not sure, but what if you couple the N-terminal of an amino acid ester (t-Bu or Me) with DCC/DMAP, to the carboxylic acid? These reactions run under mild conditions. However, removing the alcohol moity as a protecting group might give some problems, as amides hydrolyze as well..

You can also react the acyl chloride of the carboxylic acid with the sodium salt of the amino acid. (for example reaction of glycine and benzoyl chloride yields Hippuric acid in good yield)

You might consider using pentafluorophenol as a leaving group too.