Used a book of building here to skip 14 days on this tesla upgrade. I will leave a bunch of these teslas and xbows at TH11 level (lvl9 and lvl5 respectively) to efficiently use books on. I will plan accordingly once all my other defenses are “caught up”. Most likely, I think I will work on 2-3 TH12 upgrades while upgrading a few weak defenses. This allows a good balance between long and short, cheap and expensive upgrades.
Working solely on TH12 defenses is boring and tedious because they cost 12mil and 14 days. This means an average of one upgrade per 3 days (2.4 days with gold pass) and ~4mil gold/day. If I can slot in some low level defenses, it would drastically reduce gold/day and I can work on cheap upgrades on a near daily basis.
Few gold troops now including rage spell. I used a book of spells on rage spell lvl6 off-screen because it was the longest upgrade available. I figure that a 3% boost to electro can sometimes be really crucial. I made a small mistake here by upgrading pekkas a few levels before moving to freeze spell. You should try to focus on one troop until it is maxed. I thought I wanted maxed pekkas for pekka smash but thought my freeze spell needs more attention and would be more useful
Used BoH (book of heroes) to skip 7 days of BK upgrade so BK is now lvl54. Unfortunately I don’t have enough DE to upgrade him again. In that sense, I did not skip 7 days of BK upgrade bur rather it depends on how fast I can farm. With the gold pass, I only saved 6.3 days of BK upgrade.
If I can farm 190k DE in 1 day, I only saved 5.3 days of upgrade time which is still great. If however, it took me 3 days to farm the DE, I would only save 3.3 days of BK upgrade time. Since you can buy BoH for 500 gems in the trader, you can see that it is only worth it if you can farm quickly. If not, you may even save more gems buy just gemming the BK outright (3 days = 500 gems, 1 day = 220 gems)
Spoiler: it took me about 1 day to farm the necessary DE for BK.
This final picture above was taken the day after using all the books. BK going to be upgraded in 1 hour so the book was well spent.