A creature attacks alone if it's the only creature declared as an attacker during the declare attackers step. For example, Heiko Yamazaki's triggered ability won't trigger if you attack with multiple creatures and all but one of them are removed from combat.

You must still pay all costs and follow all normal timing rules when casting an artifact from your graveyard this way. Normally, this means you will not be able to cast that card until your post-combat main phase.