Many difficult fights become easier before the first attack happens. Positioning, resources and environmental awareness can matter as much as character builds.
1. Look at the battlefield first
Check elevation, chokepoints, hazards and routes before committing the party.
2. Know which limited resources are available
Review spell slots, abilities and consumables so you do not discover too late that the party is already depleted.
3. Put each character where their role works
Frontline characters, ranged attackers and support characters should start where they can contribute quickly.
4. Change positioning before rebuilding characters
If a fight repeatedly fails, try a different approach to space and target priority before assuming the whole build is wrong.