The only one that I found difficult on "Tough" difficulty solo was the final one in the hole with the raised ring ledge all the way around the top where 3 goliaths come in to melee the beacon, but I figured out a good way to do it solo:
What I did is beforehand go to a nearby peacekeeper outpost to the southwest of the research center it I think, I armed myself with the hmg machine gun, flock laucher, and cluster grenades (clusters are important). The key is to get a minigun turret off of the outpost ledge. there's 4 turrets on top of the wall over the road going thru the wall at the outpost (I think the road leading to the gate in the wall there is a small bridge over water). You have to melee the agent off to take the turret but it's just one so the guy doesnt get too pissy.
Now just run to the beacon, go around to the front of the research center to jump up the platforms without needing to grab ledges (since you can't while carrying a turret with you). Now activate the beacon. I like to shoot some of the large piles of oxygen tanks and explosive barrels so I don't get knocked into them and insta-die in the off chance I get knocked below or have to go down there for some reason. In this strat we won't be down here at all if we can help it though. Anyway, do that quickly and jump to the upper level. Lay in wait with your detached machine gun turret on the upper level and just aim over the edge down at the big guy when he comes in and perforate him. Then, as the first wave of reapers come in on the top level drop your turret and quickly headshot all of them with your hmg machine gun until the next big guy comes in. You will want to conserve your turret's ammo because you will have to kill 3 goliaths.
Note that you can both melee AND do a power air drop slam WHILE holding the turret, so use that if little guys are crowding you or knocking you around while you're trying to perforate the goliaths. You cannot however throw grenades while holding the turret, so keep that in mind. Watch out because there are also some huge piles of explosive tanks on the upper ledge ring too, and combustible freaks can cause these to detonate - you probably want to shoot the piles of tanks from a safe distance when you have a chance. Now, since you aren't down below at all, freaks that are not actually marked as "attackers" will actually start picking away at the beacon in the lower area. I toss a cluster grenade or two down on opposing sides of the area below to clear everything down there out as they build up, but I never leave the top circular ledge.
Going downstairs is is just bad. You can't reliably kill the reapers spitting from the top because you shoot them and they fall backwards before they die out of your line of sight and take a while to get back up, wasting time before you can actually kill them and more importantly you'll get charged all to hell by pounders if you are below and that is very likely to get you killed because of how long you can lose control of your character when you get knocked down. Not to mention making you drop your turret.
Now, back to the strategy working from the top ledge: Always drop your turret in between goliaths and kill the top tagged reapers with the hmg as fast as they pop, and make sure you dont let the ones down below build up too much. I really, really suggest NOT using rockets of any kind because if a freak hops in front of you as you fire a rocket turret rocket as some people recommend, or a homing missle, or god forbid a flak round you will blow yourself to hell. With the minigun there is no such risk and it chews up the goliaths very quickly.
Overall: just don't panic, snipe with your hmg on the marked guys on the top level, and as you get crowded by scroungers etc do a couple power slams to get them off your back while periodically throwing cluster grenades down below to clear out the little guys. It took a couple trys to figure out the best way to handle all that, but once I started using the turret to only kill the goliaths it went much smoother because the minigun doesn't work very well as a sniping type weapon to take out the tagged reapers and you can't quick lock on to things like you can with the hmg.
Also, if you run out of clusters or something and have to use the flock gun, JUMP and only fire downwards while in the air. you do NOT want to accidently cause an explosion in your own face. there are a ton of barrels around and you will be getting hit a good deal by little spitters etc. You'll have to kill 3 goliaths total, and by the end of the charging you'll be frantically ground pounding and sniping down tons of tagged reapers (use the hmg!!) and clearing out the area below with clusters and, if you run out of clusters, flock rounds work pretty well shooting them down into the pit area, but it shouldn't be necessary. I did this solo and I dont think the beacon got below 60 or 70% health following that strategy. I hope this helps you because this one is definately pretty freakin nasty.
EDIT: jesus christ that was long, but I hope it helped some people. I really need to figure out how to make good in-game videos without filming the screen with a camera, because I don't have one. :p If anyone knows a good way to do that shoot me a PM! I love posting solutions, but I tend to get exhaustively descriptive so they tend to be quite long, but if a picture is worth a thousand words, then a video ought to say even more, which is what I'll need :p