Chapter 1: A Stranger Comes to DagdaMinor spoilers
Place Fabio beside the target to lower Dodge, then initiate with Dietrich to stack Murderous Intent. Keep inherited EXP bonuses off until after changing class.
Place 
Fabio beside the target, then initiate with 
Dietrich to stack Dark Calling and Murderous Intent for a higher critical rate. Diadem of the Endless has only a 10% chance to restore HP. Do not plan engagements as though every attack will heal you. If you inherited an EXP bonus when changing routes, turn it off before battle and reactivate it after changing class to avoid leveling too quickly beforehand.
Turn off the EXP bonus until after changing class
If you inherited a purchased EXP bonus when switching routes, disable it first and turn it back on after changing class. Lower-tier classes have smaller growth modifiers, so minimize levels gained before the class change.
When allocating Karma Shards, reserve 1000 for the class EXP bonus before considering other purchases.
Place Fabio beside enemies to stack critical bonuses
Before initiating with Dietrich, place Fabio adjacent to the target. Dark Calling lowers adjacent enemies' Dodge by 5; Dietrich then initiates to activate Murderous Intent's Critical +5.
Before engaging the next enemy, calculate how much damage Dietrich can take using his current HP. Diadem of the Endless can restore his HP, but it triggers only 10% of the time. Do not count that recovery in advance.
Dietrich
Fabio