Use your first Limit Break to claim this trophy, which has to happen in the Materia Guardian boss fight in Chapter 1.
Fledgling Summoner
Invoke a summon.
First, be sure that your characters have Summon Materia equipped. In some prolonged battles, especially those against powerful enemies and always against bosses, once you Pressure and/or Stagger them, a Summon bar will begin to fill above your characters’ HP bars. Once full, that indicates your characters can spend 1-2 ATB charges to invoke the Summon they have equipped. Doing so will award you with this trophy.
Team Player
Use a synergy skill.
Unlock Synergy Skills in character Folios marked by red nodes with a target symbol. Then, in battle, hold R1 to guard and bring up the Synergy Skill menu. These free team-up attacks have varied effects and uses, so successfully execute one to claim this trophy. Cloud and Sephiroth do have a Dualblade Dance Synergy Skill that can achieve this in Chapter 1.
No “I” in “Synergy”
Use a synergy ability.
Like Synergy Skills, Synergy Abilities can be unlocked in Folios via red nodes, but the symbol looks like two arms curled together. These major team-up attacks are on par with Limit Breaks, and both characters have to use other abilities to generate 3-4 Synergy Charges before executing them. Sephiroth and Cloud should perform the Synergy Ability Double Helix during a Chapter 1 boss fight against the Materia Guardian.