This isn't necessarily Chapter 2 specific but prob first 5 chapters in general. First thing you should do with your diamonds is to upgrade your Magic Academy. Then you want to craft Vallorian Valors (gator guys) from the Magic Academy. You won't have a decent unit to go up against heavy range except Treants for awhile. Unfortunately, Treants like to take scenic tours getting to the battle scene. Vallorian Valors at least have one extra range to reach the enemy heavy range slightly faster. If you can reach them faster and kill them faster, it's less hits your other light melee and light range have to take from enemy fire. When you enter a chapter, sell off the old Vallorian Valors and re-craft them in your MA. Don't waste Royal Restorations on them. While you're in the Magic Academy, also keep an eye out for Bear artifacts, Brown Bear base, pet food, and military buff buildings (Enlightened Light Range, Magnificent Mage Multiplier, Unleashed Unit Upgrade). Brown Bear gives a lot of units passively that you can't train on your own until pretty late game. I would save the buff buildings for later chapters when you can fight deeper into Spire/Tourney, but you can never have too many and the chapter of the buildings does not matter.
The troop production in early game is terrible so always max out your barracks ASAP. It's very easy to over-extend your troops early game. When you unlock Needles of the Tempest, get that puppy down. It speeds up barrack production and buffs your archers and other light range units.