Reinforcements can only be earned while playing as Storm.
Tempshields are non-reloadable Support Weapons. There are four variants of tempshields:LV 17 TEMPSHIELD A1LV 42 TEMPSHIELD A2LV 67 TEMPSHIELD A3LV 92 TEMPSHIELD A4
Tempshields act as throwable grenades, only instead of inflicting harm they boost players' and vehicles' resilience to damage for the mission's remaining duration.
Players subject to a tempshield's effects thus become "Reinforced".
It matters not which variant of the tempshield a player decides to use to earn the trophy.
Although players would boast multiple tempshields in their inventory upon a mission's beginning, only one tempshield reinforcement counts per partner affected per match. Meaning spamming multiple tempshield-throws at the same partner would only count as one reinforcement per match.
To quicken the pace of reinforcements, players may opt to play either "Online Cooperative" or "Local Cooperative" with up to three other partners; enabling up to three trophy-valid reinforcements per match.
Multiple players may gain trophy-valid reinforcements on the same mission by simply throwing tempshield at players that had already been reinforced. Although the effects of multiple tempshield do not stack, each respective match-participant would get a single a reinforcement tallied. There is no need to take turns between players seeking the trophy.
Players should note, however, that for a reinforcement to register the mission must be won. Restarting the mission via the "Restart" option on the "Mission Over" or pause menu would reset any trophy-progress that had been done during the mission. 
Self-inflicted tempshield reinforcements do not count as, nor dawdle any, progress toward the t...