It is recommend that you start your playthrough on Hard, since it's unlocked right away and the difficulty achievements are stackable. This difficulty isn't much different than playing the game on easy. When you have enough Energon focus on upgrading your health, your vehicle shield, your shield regeneration and anything else that will protect you from damage. Also upgrading your primary weapon will help out a lot. I personally like the Neutron Assault Rifle, it's a very common weapon, and the Riot Cannon, a very powerful and useful gun. There are no real boss battles through the game so no worrying there. Remember to not rush into areas. Stay back and use all your resources to your best.
Easier Method for Hard:
If you want to enjoy the game, grab all the collectibles and get the related level achievement, then play on Easy. Once you have completed the entire game on Easy, gathered all collectibles and gotten all level related achievements you can now go and use chapter select to replay any level.
What you will want to do is play through the level as normal on whatever difficulty you like. Then when you reach the final check point, simply Quit on the last checkpoint of the chapter > change the difficulty to Hard (for that specific chapter) from the Main Menu, by pressing > then select Continue from last checkpoint. By doing this the game will recognize that you have completed that chapter on Hard.
NOTE: This method does not work by changing the difficulty when you die. It must be done from the Main Menu.
Chapter 1: When you're crawling up towards Optimus and Megatron.
Chapter 2: When you get off the artillery gun and state it needs more Energon, you start descending into an unknown territory.
Chapter 3: After your help is destroyed and you cross the bridge, before entering a door you have to pry open.
Chapter 4: When you have to escort the bomb.
Chapter 5: When you fight a couple of waves of Bug Bots, then there's an escape sequence. (When you automatically transform into vehicle mode, quit here.)
Chapter 6: After you defeat the Titan, you will have to plant a bomb to escape.
Chapter 7: When you have to fight two Titans. (You will need to fight them on Hard, level ends after both are dead.)
Chapter 8: Right before you break into the control room.(As soon as you play as Bruticus, quit here.)
Chapter 9: After the arena wave battles that comprise this entire chapter, there's a save section to quit out at. (I quit out at this point, but when I loaded it back up after the wave battles. It made me fight the 5-6 Leapers again on Hard. Not bad since you have a Riot Cannon! and at this point you should have it fully upgraded.)
Chapter 10: When Soundwave is working at a computer to power up Trypticon and you have to defend him with help from a few turrets.(This sequence must be fought on Hard).
(**Make use of the Diffraction Barrier and the Thermo Mine Payload, they respawn again after a few seconds.)
Chapter 11: After the fight with Hardshell, there's a safe sequence to quit out before the level ends, before you cross the bridge, quit here.
Chapter 12: There is an escape sequence when Grimlock transforms into hes Dino mode and you have to escape before the time runs out.
Chapter 13: When you are asked to make your choice.