Builder's Base Strategy - BH6 Upgrade Guide
BH6 adds some new toys as you'd expect - there's a third Archer Tower, a second Crusher, and, of course the Roaster. You also get 4 new wall pieces, one mine, and one MegaMine.
Step 1 - Drop everything that you get new at BH6.
Right away, your initial focus needs to be getting the new defenses on the game grid. This is always the case when upgrading your BH because the new defenses will put you ahead of those you left behind at the previous BH level.
Step 2 - Make the new stuff match the old stuff.
This is also a standard step for my games - I upgrade anything new to match their counterparts from the previous level. In the case of BH6, you'll want to bring the Archer Tower to level 5 and the mines/MegaMine to level 5 as well. Get the Crusher up to level 4 (level 5 is where it gets expensive). I'd make this the last of the new items to upgrade to match existing, though. The Roaster can wait a little bit.
One of your new BH priorities should also be getting the Star Laboratory upgraded as soon as you have enough elixir saved up to do so. Ideally, this will be just a day or two after you've upgraded. This is important because you want to be working on troops as soon as possible.
Step 3 - Shore up air defense.
The game meta strongly favors air at BH6 (BH5 too, for that matter) so I start there on the upgrade path. First, get the FireCrackers to level 6 followed by the AirBombs. Once they are done, move on to the first three levels of the Roaster, then split back off to your Archer Towers.
In between, hit things like mines and traps. Since the level 6 upgrades take a few days to build up resources for, and mines and traps are cheap, I like hitting them along the way.
Step 4 - Strengthen ground defenses.
Once your air defenses and Archer Towers are solid, move on to what's going to hurt your opponent's ground strategy most - the Double Cannons. These are particularly effective against the Battle Machine.
Next up, get the Hidden Tesla to level 6 followed by the remaining three levels of the Roaster, then Cannons, the Crushers, and the MultiMortar. I usually leave the Guard Post to the absolute end; you don't get a whole lot for upgrading it, although I guess taking the troops to level 12 is worth it on the long.
Assuming your Battle Machine is at least level 5, I suggest putting that off to the very end as well. There are far, far better places to put your elixir and the BM uses up 8.5M; even at max loot, that will take a few weeks to knock out. A better use is troop upgrades.
Step 5 - Everything else.
Once all these are done, THEN I would hit the Clock Tower, Gem Mine, and maybe walls if BH7 hasn't been released yet.
At BH6, your best troops are Beta Minions, Sneaky Archers, Baby Dragons, and Night Witches. Level 12 Night Witches absolutely rock the game, especially when paired up with a couple-three camps of Beta Minions. Level 12 Beta Minions and Baby Dragons provide extra troops in your Army Camps, and the level 12 Sneaky Archer has what will seem like hours of invisibility cloak (ok...maybe it's only 10 seconds, but that's an eternity in this game). So....
1) Beta Minions
2) Sneaky Archers
3) Baby Dragons
4) Night Witches
8) Cannon Carts (after you get your Battle Machine to level 10)
I don't recommend upgrading your Battle Machine until closer to the end of your time at BH6 because there's just not enough bang for the buck at level 10. It's been suggested we'll have TEN levels to upgrade at BH7 and some players are making certain they get their BM done before BH7 is released. Of course, we have no idea when that might happen so I suppose I'm just kinda rollin' the dice, livin' la vida loca....
When is the best time to upgrade the Battle Machine...? This depends on your personal preferences. Myself, I say keep pouring that elixir into your troop upgrades until everything but Cannon Carts are at level 12. If you're rushing, or BH7 is released - I'd probably put it right after Night Witches. Either way, I'd definitely do Night Witches before the Battle Machine as they open up a lot of attack strategies.