Always Hard - this one cost me a lot of hours!
This isn't a solution as such, because there isn't an easy way to do it - you have to be good enough. I can say how I got it though, after owning the game for over a year.
As a level of skill, if you can't 5 star all songs on Medium I don't think you're going to succeed at this.
After completing Medium and getting 5 stars on all songs, I elevated to Hard. That orange button was difficult at first, rather than stretching your pinky all the time, learn to move your whole hand. The yellow button has a raised notch on it so you can tell what buttons you're on by feel.
By the time you get to tier 5, go back and retry some of the songs on tier 1 and 2, you'll probably find you can 5 star most of them, or at least get higher scores than you did originally.
Baracuda (tier 2) is one that troubled me for a while - you have to use up/down strumming for the 3 note main riff, and my rhythm sucked.
You'll progress to tier 6, get past those and then find that tier 7 is a whole new world of difficulty. Try the songs, although at this point it may be worth starting a career on Expert. You will probably be able to get through the first 2 tiers at least by now, and playing on expert makes hard seem easier when you go back to it.
When you're just trying to beat the songs, save your star power up for the bits you get stuck on.
Tier 7 songs:
Before I forget - Practice the main riff using hammer-on's where you can, up/down strumming will be necessary on the quick notes, and I found for the chorus holding the chord and moving my whole hand worked far better than trying to change my fingers over.
Stricken - Hammer-on's are definitely necessary for this, there's no penalty for strumming a hammer-on though, so it's worth strumming every few notes in case you miss one. Same thing as above moving your whole hand for the stepping up chords.
3's and 7's - This one held me up for a long time. Up/Down strumming followed by quick chord changes.
Knights of Cydonia - Brilliant song, difficult though. Even if you can't perfect the really fast strumming part you should only miss a few notes so you should be able to stay in the green until the galloping triplets section. Save all of your star power until here, then get good at hammer on's fast! The final section I find is best played with up down strumming, it's really upsetting if you fail here the first time you get past galloping triplets!
Cult of Personality - again with the hammer-on's. Lot's of solo in this, but they're reasonably spaced out for the most part. You'll probably 5 star this before the other songs on tier 7.
Tier 8 songs:
Raining Blood - play it once, get to Mosh 1, and fail miserably. I may have sworn a lot at this song! Move on to the other songs, come back to Raining Blood.
Number of the Beast - up/down strumming is helpful on the intro section, but as the other solution says the odd timing will have to be mastered. The chord sections are great for getting more points and back in the green, on your first go I'd save star power for the solo.
One - The first 700+ notes are well spaced up and at a reasonable speed, so shouldn't cause a problem. Next is the 'drumming' part, up/down strumming is required. Save your star power. Solo A is the fastest solo, if you can do it without missing notes you're better than me, strum regularly to ensure you don't miss too many notes. Activate star power when necessary here. Solo B often gets me as well, they're not hammer-on's so up/down strumming helps. Solo C and D I failed on before because I was still strumming too fast, try and calm down and play what's there. If you manage to get star power again activate immediately to get your rock meter back up again. Don't expect to beat this quickly, it will take several attempts.
Cliffs of Dover - The intro and outro are the fastest parts, the rest of the song is fairly regular. Probably the easiest in Tier 8, but by no means a walk over.
Raining Blood - now you've done all the other songs, it's time to come back to raining blood. Go into practice mode and slow down Mosh 1. Gradually increase the speed as you get better at hitting the notes. Playing more songs on expert may help, but this on Hard is harder than most songs on Expert. You WILL get to the point where you can beat Mosh 1 (even if not every time you play it). Trying to get 5 stars on Baracuda will give you practice on the triple greens. When I was playing if I didn't nail the star power sections before Mosh 1 I'd just restart. After Mosh 1, it gets a bit more repetitive so easier to hit. Try and save star power for the 'Flood' section, although use it if you're failing.
Battle with Lou - Prepare for frustration. You've finally beaten raining blood, and you go into the battle with lou. You've learnt up/down strumming on quick notes in Knights of Cydonia, you've learnt triple note strumming in Raining Blood and baracuda, you can do this! By the time you get to the first 'star power' section you should be well in the green. But don't use your star power! Lou is so good you'll need to hit him with 3 attacks in a row. And yes, that means you have to survive at least 3 of his attacks while you're building them up.
This took me more attempts than Raining Blood. By the time I finally beat him, I had 5 stars on all but 6 Hard songs. I was up to tier 6 on Expert, with 5 stars on at least half of the one's I had played as well. Ultimately playing the song over and over is the best way to get better at it, and you will beat him eventually. Good luck.