Ok so I completed this on hard, so my advise is based on my play through at this difficulty.
There are effectively 4 stages to this - restaurant, casino, music room and boss. All are static arenas where you have to survive wave after wave of enemy.
For the restaurant, what I did was go to the opposite of where you start, at the bottom of the stairs. Put the guitar cases either side on the ground floor. From here if you stand in the middle you can see the enemies coming left/right or from the top. Use RB for bullet time rather than diving about as this way you can slow time, headshot, and then resume time to save as much as possible. Use guitar cases when needed. The hard bit is when the boats come in as these guys take more damage. I used the same method but with the assault rifle. When there is only about 1 guy left, move to the other side where you came in - the reason is the lift opens where you are and some rocket dudes come out. Take these out using the same process.
Next level is the casino - for here I just put the guitar cases in the area you start (before the casino), stand back and switch bullet time on/off as they come up the stairs bit to you. This wasn't too hard.
The next area for the band I found v hard but think I found a good spot. When you start look right at the bar - go to it but hug the wall to the left as you face it. From here most of the enemies will come at you from the door very opposite, so you can shoot from range. If you hug the wall, you can angle it so even enemies who come round the corner walk right into your crosshairs. Save bullet time for the harder dudes. I managed to do it here needing only 1 health pack.
Last up is the end of level dude, and it took me 1 1/2 hours of trying to do this. First back up a bit and take out the statues so they hit him (this seems v hit and miss!). If he takes enough damage you get a cut scene. From here go right and collect enough cranes to use barrage. Use it and unload on him and you should do enough damage to trigger another cutscene. Work your way around again to pick up more cranes so you can barrage him again - you should be able to kill him this time!! This way you avoid the need for going upstairs or timing shooting when he is reloading. He does so much damage you can't face him. However you only get enough cranes for 2 barrages, so this strategy only worked for me when the statues got him down the first third health! Good luck!