I assume you are on a Playstation 2 with normal controls.  Open the
command window for any of your characters (the thing that says Attack;
Magic/Techniques; Mist; Gambits; Items).

Select Items and then select Elixir (I assume you have some of these
in your inventory).

At this point you can give the Elixir to one of your three onscreen
characters.  If you hit L1 or R1, you can change the list to choose
either one of your other three offscreen characters or one of the
enemy fiends.  You may have to hit L1 or R1 a few times in order to be
able to select the magic pot to be targetted with your Elixir.

[This is also useful in other parts of the game.  For example when
fighting a mark with an Ally, you can use this to target friendly
magic towards the Ally.  Also, if a character is knocked out, you can
switch them out of the party and then use someone in the party to
resurrect them.]

This should be enough info.  I apologize for inaccuracies.