Its very easy to fall into the trap of assuming characters can only do one thing, and while its true that they have specializations, the card pools are large enough that any character can go into any position! Be creative! Good composition construction starts by assigning your heroes roles. The simplest and most generally successful is: Blocker, Healer, Support, Carry.
Hero decks are at their strongest when hyper specialized. It’s tempting when adding cards, to take ones that do not specifically fit into the pre-assigned role. However this is almost always an error. This game punishes Jack of All Trades heavily and allows heroes to cover each others weak points, instead of demanding they all perform on every metric. Keep your hero decks trimmed and good at doing the one thing they’re trying to do.
Blockers
Generally best done by warriors. Blockers love going first so they can provide Frontline support, high resistance, and a large hp pool to take plenty of “front” attacks. Blockers protect their team from damage, lessen the load on healers and prevent heroes from being oneshot by ultra powerful enemy attacks. Block cards like Barricade, intercept and entrench are particularly good on rate and are always playable. Blocking is not always necessary so a blockers deck should heavily feature support cards that can be played instead. Examples include helping hand, sharpen, howl.
Healing
Generally performed best by healers. Being naturally slow is a positive trait here, a well-timed turn order will have the team taking damage, then immediately following up with strong heals. Heroes cannot be healed past max hp, so healers frequently end their turns with a lot of excess energy. Instead of holding that energy for later turns to heal more, it is more desirable to use all mana every turn. Cards like Divine Power are exceptionally strong on healers, because they turn excess mana into ultra strong bless for our carry. Additionally, a healer deck only requires a few particularly good cards to top up a team. This means most strong healer decks end up being primarily comprised of non-heal cards.
Supports
Mages love this position. Supports ‘donate’ their energy and card draw to other players and apply critical buffs and rebuffs. Cards like scroll of intellect and fortune telling provide utility to the team, and when aimed at the carry will generally be stronger than anything that character can do by themself.
Carry
Carries provide nothing to a team aside from damage. They are the recipient of most buffs from the other heroes, and aim to have strong popoff turns that blow through enemies health by playing full hands of strong attacks. Carries like going last in the team so they can fully benefit from team buffs.