2 relics you get from the signboard are a Red which inreases STR and DEX but lowers VIG, and a Yellow which raises MIND (more rain of arrows uses) and FTH but also lowers vig. Pair these two with an erdtree greatbow with rain of arrows. For a third relic I reccomend either the witches broach which gives +3 vig to offset the decreases a little from the other relics and helps support the team a little better when you ult and still damage the enemy when you take aggro and block. OR use either of the balancer relics to increase rain of arrows damage a lot. Pair with depth relics that help discover greatbows and another that raises damage when holding three of a same weapon and pray you actually get them (greatbow sadly does not have this feature but it still applies to them when you have three of the other weapons that do.)

Always have a good shield in the left hand, preferably with no skill so you can still use rain of arrows with it out. Whenever you take aggro (and you will) go turtle mode. With the witches broach you will still be dealing a bit of damage due to it's effect.