Gawain uses a skill to increase his NP Gauge and apply Guts for himself. Use an Archer servant which has a NP to deal damage to a single enemy to defeat Gawain
This is the hardest battle among Tristan Interlude Quests because Enemy Tristan gains various Buffs in this battle. Use Saber servants instead of Lancer. Use skills which apply Invincibility for your allies to avoid Tristan's NP. Check the table below for Buffs Tristan has.
Before Break
Gift "Reverse" Tristan (Enemy)'s Class Matchup changes (Tristan's ATK is effective against all the classes with except of Shielder, and Tristan is weak to Saber class servants' attack) & Apply Immune to Poison for himself.
Post Break
Gift "Reverse" Tristan (Enemy)'s Class Matchup changes (Tristan's ATK is effective against all the classes with except of Shielder, and Tristan is weak to Saber class servants' attack) & Apply Immune to Poison for himself.
Gift "Nightless" Decrease NP Strength & Increase NP Gauge by 2 each turn for himself.
Gift "Fierce" Increase Resistant against various attacks for himself.