Passing (for short passes strategy), crossing (for long passes strategy), or both (for default passing - but only if your player is la7+) Other important skills are: technique, creativity, teamwork (importance is in order that I wrote)
These skills are for amc, amr, aml.. Less important, but very useful skills for attacking midfielder are: of the ball, agility, acceleration, pace.
For me, great assistant player should have all of these 15+
But if you have la6 (or less) player, you should focus on training these skills up to 15 - in order that I gave you.
Greetings.