Campaign levels are like puzzles, and typically require use of specific units and/or upgraded units to succeed - especially if you are seeking a 3 star result. A few general tips for success -

• Use Tier 1 troops (barbarian, archer, goblin) in hordes to attack single target defenses (cannon, archer tower, spring traps). You'll loose a few, but the horde will overcome.
• Use Tier 2 troops (giant, wall breaker, balloon) for their
  specialized abilities, particularly to crack splash damage defenses
  like the mortar, which can wipe out your hordes. When using the
  balloon, be sure to wipe out air defense first.
• Campaign levels are not timed, so once defenses are down you can wait indefinitely to win if you want to save troops.
• Queue some troop production in your Barracks before starting a fight (even though you are full). Some of these troops will already be ready and waiting as soon as you are complete.

For per level strategies, check out the level by level video guides here.

I will also add that upgraded units help a lot. As I now attack the high-end levels, it is much easier to 3 star now that I have most units upgraded to level 4 or level 5. I also make use of the Barbarian King, which I believe makes these levels much easier since he is a very good tank, and can draw a lot of fire while your hordes disassemble the defenses.

All builds are different but in my opinion you should not be working on your walls at all at this time. If you face any enemy that is strong enough to blow past your defenses (cannons, archer towers, etc), they will get past your walls.

TIPS at Townhall Level 5

At this TH level, You should try and focus on farming & building. I do not recommend going for trophies or otherwise known as cupping. The more trophies you have, [99% of the time] the harder the opponent you are trying to farm from.

Try to max out your camps, barracks, and Clan Castle (which you also have to be in a clan) first so you can have the max number of troops to attack / farm with. At TH level 5, these are the max building levels:

At townhall level 5, max level barracks is level 7, max level Army Camp is 5 and max level Clan Castle is 2.

Note the unlocked units at Barracks level 7. The strongest unit you have is the Wizard, although you should factor in cost of elixir per attack.

As the picture above shows, you can have a max of 3 level 5 army camps which will hold a total of 135 troops. Take the 135 troops + 15 troops from a full clan Castle (which again you must request from clan members) = 150 Total attacking troops.

The last factor you have to put into perspective is what league you are in. My recommendation from average opponent strength would be these builds:

TL;DR

Trophies 0-1400

• 10 Giants (housing space of 5 each) = 50
• 50 Barbarians (housing space of 1 each) = 50
• 8 Wall breakers (housing space of 2 each) = 16
• 29 Archers (housing space of 1 each) = 29 OR 29 Goblins (housing space of 1 each)

Total of 150

If elixir and time is not a factor when farming, then a good tank & heavy DPS build should do the trick.

• 15 Giants = 75
• 10 Wall breakers = 20
• 12 Wizards (housing space of 4) = 48
• 2 Goblins/barbarians/archers = 2

Total of 150

Have your Giants be your tanks. They will go for defenses first. While the defenses are distracted by your tanks, try to attack their resources.

As I mentioned above, I do not recommend going for trophies. Once you have the majority of your opponents resources, quit the attack by tapping the Surrender button.

The more gold you have, the more you should focus on protecting your resources for when you are not playing. Therefore, I would advise putting your gold back into upgrading your defenses such as cannons, archer towers, etc. Try not to upgrade your walls past level 5 as you should put those resources elsewhere.