i'm not sure if it's outside acre but i think so, but yeah go to that camp, run around get a whole bunch of them together and just kill them all....there's also a whole bunch of king richard flags there
When u hit mem block six and go to the place after robert you will unlock these achievments very easily, just ride your horse through the encampments and let the gaurds chase u and counter kill the lot of em
The 10 minute achievement is incredibly easy, although boring. All I did was get to a fairly secluded area that had some wooden scaffolding (or pretty much anything that you can climb and the enemies can climb as well, but it can't be to high, as long as you can throw a guard off of it and they live, you are ok). I got one guard after me and I then climbed up the wooden scaffolding and the guard followed me up. As soon as the guard climbs up, you grab him and throw him off, they will get back up and climb up after you again and you repeat this till 10 minutes is up.
Essentially, the "rubberband the r trigger for 10 minutes" will almost always work, no matter who you are facing, as long as you back yourself to a wall. Also, be quick on on grab breaks if you are facing tougher guards.
Also, using your vigilantes to hold your attackers as you parry up and wander around through the little chaos that is present, is a proven style.
No matter which technique you use, having "complete symbiosis" or a full health bar will help a lot, you will mostly likely take a few hits no matter what, but you will most likely regain the bars quickly. This shouldn't end up be a big problem no matter what though.