Hi I’m upgrading my bank guild with a friend and whilst it is hard its not impossible.
We claimed our guild hall with three people, we couldn’t quite do it ourselves(But I’m sure its doable) but a third tipped the balance and made it easy.
Single target bounties, any guild race(Just turn up when a big guild is doing it and complete one run) and 5 target guild treks allow you to build up favor easily enough. All of these you can do solo.
The rest is all money.
Give me a message in game if you want an extra hand taking the guild hall or ask in Lion’s Arch for volunteers.
Good luck