Summoners need Charge dice in order to summon their Eidolons.  The base cost is 1 CD, for a level 0 Eidolon (the base).  For every level an Eidolon goes up, it costs one extra CD - so a Level 4 Eidolon would cost 5 CD.  You want to stop a summoner from bringing it's friend?  Kill it.  Quickly as possible.