Locking doors does keep out Scientists and some Specialists iirc. Also, if a Warlord or whatever unlocks a door, he won't go through the door immediately afterwards which can be extremely helpful for pacifist completions, which aren't actually that difficult. It's not that hard to complete a post-game facility with 100+ cycles without hurting anyone.
I think it should stop you from reacting elements you've already reacted. It's tedious, especially when you accidentally drop Man on itself for the fourth time and waste fifteen seconds closing the friendship screen.
Getting the hard badge is not actually all that difficult. The trick is not to fight hand-to-hand if possible - especially at higher levels, it's almost impossible to maintain the kind of damage ratios you need. Early on, you can rack up several free kills - kills 1-3 can be grabbed sustaining minimal damage simply by wind-up punching. Then, you should focus on items that can get you free kills - the pitchfork is an overlooked one (you use the secondary to hook them, then release them over the drop). The sniper rifle, taser and shurikens are also good examples.
Another tip is to make sure to pick up as many weapons as possible. Even if you're holding something better, the opponent is going to make life hard for you if they pick up that blowdart gun or those mines. You can exploit the fact that they go for the crates, as well - wait for them to do so, then use the space to get a kill using the bazooka or a grenade.
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