Before you collect the minikits, purchase Saruman (he's in Rivendell and costs 50,000 studs) and Sauron (he's in Dol Guldur and costs 70,000 studs or enter the code “OARA3D”). It has to be night time for both, so go to a campfire and change the day. The reason why you want to unlock these two is that some minikits require someone who can break Mithril bricks (Saruman) as well as black bricks (Sauron).
To help you even further, get the Minikit Detector red brick if you can afford it as soon as possible. When turned on, this red brick indicates on-screen arrows where exactly the minikits (or tasks to reveal the minikit) are located in each level. You first need to locate the Mithril Bandit Gloves schematic.
Schematic Location: In Lake-town, look for a building you can enter near the center of town with a basement. There’s a cage and a hook point to the right. Use the hook point with Dori, grab the crank, and attach it to the mechanism to the left of the stairs. Crank it and step on the pressure pad behind the raised cage. This reveals a target. Shoot the first target with Legolas, then shoot the second target with Ori three times. Once the second gate opens up, slingshot the targets so the left is red and the right is blue. A chest with a key will appear. Grab the key and open the chest, revealing the schematic.
Red Brick Location: Take the schematic to Bree and forge the item. Then you have to give it to the woman who gave you the “Spoils of War” quest in Thranduil’s Kingdom. On the map, look between the words ‘Rivendell’, ‘Erebor’ and ‘Esgaroth’ (in the center of them) to find a deserted area near the mountains (next to the "Barrels Out of Bond" chapter sign). Set the “Spoils of War” green exclamation point as your destination and fast travel to Thranduil’s Kingdom in northern Mirkwood. Follow the gold glowing studs to your destination.