A lot of Wyll’s power is his imp. Its invisibility means that it never rolls initiative with other characters. On your first party member’s first turn, have the imp fly up to any opponent and sting them. That’s 1d6 + 1d4 damage at the most strategically advantageous spot at the battle’s onset, and 1d6 + 1d4 every turn thereafter. Combined with hex and the invocations mentioned in previous posts, that gives Wyll the highest damage potential in your party : 1d10 + 2d6 + 1d4 + cha. Not bad, especially since it costs nothing but actions.
Hex costs a spell slot, but the imp can help. Every “morning”, summon the imp, hex it and kill it. (The fastest way to kill it is to summon a new imp). Now, Wyll has Reapply Hex as a bonus action. Use Wyll’s second spell slot with some long term buff spell, such as Armor of Agathys, and then use a short rest to replenish Wyll’s spells. Voilà! The imp effectively gave Wyll an extra spell slot to cast Hex during combat.
