I activated tier mode as soon I hit level 30, but on the end I only hit tier 20. Replaying missions to achieve tier 1 is boring, so I suggest newgame+ for tier character.
Character should have activated tier mode to start this NG+. In NG+ you can't deactivate tier, but you retain your equipement, skills and tier level.
This way achieving tier 1 will be fun, not like this boring points farming we have now.