A team should have at least one healer. An ideal team is composed of 2 attackers, 1 tank, 1 support, and 1 healer. You can even get away with 3 attackers, 1 support, and 1 healer if you have a good team synergy. There will be times that you’ll need 2 dedicated healers so adjust this composition accordingly. Below is an example of a dark team for light-elemental dungeons like advents and timespace voids:
Here’s another party example for generic adventure map clearing or easy farming dungeons like Evolution, essence, Gold, or rune dungeons.
You have access to 5 party slots, in case you want to build a dedicated elemental team or you’re following a theme or some sort. You can conveniently swap between these party slots so you don’t have to rearrange your party members every time there’s a need to. These multiple party slots will also allow you to build and experiment with your party composition and skill synergy.
Heroes placed on the leftmost slot of the party becomes the leader and has their Leader skill activated. This skill is also what other players see and use should they select your leader as a helper. The rightmost slot can be fitted by another character or a helper owned by another player, even if it's a duplicate of a character you have in your party.
Finally, consider the positioning of your characters in the slot. A good example is avoid placing your healers and support right beside each other if you can help it, so that they can avoid getting damaged by certain Skills that also affect adjacent characters to the target.