Sens' Fortress is bad based on how much lewt you want out of the level.
If you just want to get to the end of it, getting past the first catwalk and the boulder mechanism are pretty much the biggest parts. Running on the roof of the fortress, trying to not get crisped, is only a problem if you're constantly going back through that area.
Now, if you want all the goodies in Sen's...that's where things start to suck a little. Because you have to keep going back through the shittiest areas, time and time again.
First off, I wouldn't even bother trying to jump down to the lower ledge in the first pit room. There's three serpent lighting bolt guys there and it's just a pain in the ass, because the fall hurts mightily and they're on top of you in seconds.
Rather, I just started jumping straight into the pit and eating the cost of 1 Estus Flask. De-equipping armor also reduces the damage you take a little. It was so much quicker, less frustrating and ultimately cost me less Estus. Assuming you have the Rusty Iron Ring, you can take out all 4 of the Titanite Demons in the pit at your leisure. If you don't have that....I wouldn't even bother.
Second big thing is: bring a ranged weapon. Sen's is full of catwalks where you have to cross while there are ranged guys shooting at you, and it's a huge pain in the ass. So I picked all of them off before I started running through guillotine blades, with a guaranteed deadly fall on either side.
Third big thing is the boulder mechanism resets on its own. I think it's trigger based, and activates when you cross from one section or another, lining up to ruin your freakin' day. Listen for the sound of metal grinding to know it's turning. I ended up having to eat a boulder every time I made a run for the mechanism, even without wearing armor I never seemed fast enough to avoid it except by a collision glitch. Plan for this so you don't waste a ton of Estus Flasks repeating the exercise.
On top of the fortress, just sprint up to the final catwalk. There's several platforms where incendiary bombs come down, but if you don't stop, they'll be targeting the areas behind you.
Once you clear the final cat walk, there's not much more to the fortress. You might want to save all the exploring and looting on top of the fortress for after you beat the boss....the incendiaries go away and you can take your time.
I spent probably 4 to 5 hours on Sen's Fortress (those fucking Titanite Demons, for some reason, I simply couldn't beat the last two. The first two? Dead my first attempts. The second two I just kept fucking up and being dumb), so I feel like I know it inside and out now. It's the kind of place where being dumb or careless means having to repeat the whole damn thing, because if you come off those catwalks you're toast.