The reduction was unnecessary imo. 2 barracks and 4 gun turrets with 2-3 engineers constantly repairing it, all built on that hill in the centre with gentler slope facing top left corner always did the trick on hard for me. Alternatively you can squeeze your whole base into top right corner, which makes defending it easier.
Are you talking about the hill which is below the center siberite and oil deposit? That hill doesn't defend any of the 3 siberite deposits and having to build your base behind it would be pretty inconvenient. It is also quite small and susceptible to a nice concave attack. However, I could see it working.
There is another hill to the top left of the above hill I just mentioned but the lower ground is towards the bottom right (attack comes from top left so no advantage here). I tried building there once but soldiers just poured through the middle forest while vehicles went via the left (surprise) and right sides.
The last hill would be the one on the top right, which is kind of horizontal and doesn't seem suitable for defending an attack coming from its left.
I ultimately decided to just build my defense on the one sole chokepoint, which is directly left of the last hill I mentioned. 2 gun turrets on each side, 2 in the middle, 2 on the leftmost side of the forest's exit and 2 barracks in between. However, I still lost but I bought a lot of time and my warehouse, labs, factory and mines were not destroyed. Only the defenses, most of my power plants and a few apeman got destroyed before they retreated.
I might give it a go again to see if I can defend properly (without the 1 minute reduction), after I'm done with Mission 8 which is also pretty tough on hard since infantry attacks always get me lol.