You might want to start by looking up existing threads on party comps and also by looking up articles on team-building and card types to get the basics down.
Generally you’re looking at the quest enemy type and configuration (when known) and building appropriately, borrowing whatever else you might want or what you’re missing from friend support.
Basic farming usually means 2-3 AoE Servants with buffing for their card types and some NP charge support in the form of CEs, Servants, and occasionally Mystic Code if desired. Batteries in their own kits are highly desirable.
Boss fights often mean one main point Servant supported by damage buffs appropriate to their main card type and some form of defense if the fight is expected to last more than 3 turns or has some gimmick.
E.g. Mordred is an ideal choice for multi-target nodes featuring Lancer enemies or a Lancer mini-boss, and you might support her with a borrowed Merlin (or a Shakespeare of your own) and level up Spartacus to help clear a wave if you can put a starting NP CE on him. Arash if there aren’t bigger Lancer enemies in the early wave, etc. Many, many options.
Enkidu is great, but mainly for single-target encounters since that’s the nature of their NP. Buster buff for their NP is common, but their deck is Quick-heavy with a very good Arts card and a semi-random Mana Burst, so they can work with almost anything to some degree. ATK buffs are safe.