Yes, when you have the skill "special", equip it, and use your mystic arte. when you see
is face appear, hit the "b" button, he'll proceed to use the arte you saw in the temple

I had him repeat the same words, ein soph auer during a combo attack, but i don't know
how.

As was said, equip skill called "special". In battle, use a high or arcane move (Marta
doesn't get arcane moves) but hold "B" as you do this, which is my hardest part (I find
using high spells easier since I use Marta (girl) and you have time to hold b. For Emil
alone, there is an additional part to the special (Ain Soph Eur (can't spell) which is
pretty useless because it is (out of all playable characters) the WEAKEST special, and it
wastes TP, but if you want to use it, press the B button again around the time that Emil's
"cartoon" face appears (the picture of Emil that is obviously drawn). And that concludes
today's lesson. Any more questions?