There are a few things I do that really help me in the games that nobody mentions on strategy threads. I will list them here.
1: Don't ever get overconfident - If you are in full iron armor with a diamond sword, don't expect it's impossible for you to die. When deathmatch comes, charging at people is a bad idea, no matter their gear. I've been in half chain with a stone sword and killed a guy in full iron with a diamond sword because he charged at the other guy who weakened him, then charged straight at me with his weakened health and a few good hits on my part killed him. Also, no matter how many arrows you have, never, ever waste them. Make every shot count. Missed shots can be picked up by your opponent and used against you for a whopping 4.5-5 hearts per hit. Just a few good shots from your opponent and you're as good as dead.
2: Be smart in deathmatch - don't charge straight at people. Hang back and let the other guys fight, or get the high ground. Maps like Breeze Island and Survival Games 4 give plenty of places to get some good high ground. Knocking the opponent off a ladder or cliff can cause serious damage. If the opponents aren't teammates, chances are the big guy will go for the closest guy to him. Hang back, wait for him to kill the other guy, and as long as the guy he killed got a few hits, he'll be weakened. Now is your time to strike.
3: Hackers and "pro" players - If you are suspicious someone is using forcefield, of course you want to use the old flint and steel/bow tactic against them, but you also have to learn how to sword fight them effectively. I find that instead of focusing on making it hard for him to hit you by spinning around, since hackers can bypass that and experienced players probably expect it, all you need to do is make sure every swing hits. I find that focusing on keeping myself pointing towards them will always let me win the battle as long as I have the better gear, and even hackers will have a hard time forcefielding effectively. This especially works if you sprint jump towards them and crit them - you get more damage, and you are knocked back enough since you were in the air to start a sprint and repeat. This allows for maximum damage against an opponent who doesn't bother with crits. (This is most effective when you outmatch the opponent with gear. The point of this strategy is kinda a "playing it safe" thing, making it much easier to not screw up, thus making it so if you have the better gear, you win pretty much no matter what, even against hackers. If you have less gear than them, than doing the regular spinning around and trying to make them miss will work most of the time, but unless you have to, you shouldn't be up against such an opponent anyway.) (This will also be effective if you can easily learn to do both at the same time, but even then it's risky since he can hit you in any direction, possibly towards a pit, etc. The less chances he has to knock you into something bad, the better.)
4: Teams - This is a more common tip, but still needs addressed here I feel. The trick to defeating teams is getting a huge advantage over them, like getting the high ground over a ladder, or possible taking advantage of a trap on the map to get some distance between you and them, possibly even kill them. The most important huge advantage, though, is separating them. A few ways to do it include: If corn is about to refill, one will likely stay there to get the stuff. Kill the guy who chases you while he is alone (use f5 to see if they are both chasing you, or if it's just the one.) and then run for your life. Chances are, they are on skype/teamspeak and he will tell his friend to come kill you. (Don't take on the other guy unless you are good on health, it's too risky.) If the guy had something you really need, like a diamond sword, then be sure to pick it up quick and get out of there. (Hiding is pointless, his friend will tell him where you are. Just get far enough away that he decides not to bother you.) You probably will miss corn, but there isn't much you can do about it. You'll have to try to come back later when your health is better. It's still a ton better to only have to deal with one of them later rather than two.
Another way to separate them is to run from them until you find somebody else on the map, and try to get near him. Some teams will split up and one will keep going for you while the other gets the other guy. Take this chance to kill the guy chasing you and make a break for it until your health is full, then take on the other one.
Finally, water battles are one of your best bets for taking on teams. The speed of you and your enemies is greatly reduced, giving you plenty of time to think, and plenty of time to regen without your hunger taking a huge dip. The thing is, knockback from fishing rods, bows, swords, etc., has all been "raised" in a sense, since in still knocks them back just as far as always, but it takes longer for them to make up for that distance. In a way, the knockback is just "stronger". This makes it easy to separate them, and once separated just a little bit, it will be easy to fight just one of them while both keeping track of the other and making it impossible for the other to help him.
5: Arrows, you can never have enough, use them wisely - Unless you know that no matter what you do, you'll get hit by one, then don't block it, try and dodge it. Dodging it over blocking it allows you to pick it up, which can be a giant lifesaver later. The second thing you can do to save them is to only use them when the opponent can't regen it. When chasing someone, if you haven't got much prior hits in with a sword, then hitting them with arrows is pointless and a waste of arrows. Arrows that miss they will pick up, arrows that hit will be regened, especially since by putting that distance between each other, you give them a chance to eat. Unless it kills them or they don't have a chance to regen it, don't bother while on the chase. Finally, if someone doesn't know you are there, do not fire arrows at them, come up to them with a sword. Hitting them with arrows let's them know someone is there, they will be scrambling around, impossible to hit again, giving them a chance to regen. Hitting them with one arrow and then coming in for the fight is stupid if they didn't know you were there prior, since a sneak attack with a diamond sword can easily do 4-6 times as much damage with crits and gives them zero chance to regen, and an iron sword an easy 3-4 times as much. Save your arrows for late fight, when they are low enough to die, but not low enough that they won't charge you. On maps like Breeze Island and The Survival Games 3, the water around corn makes it easy to get a surprise attack, run off, and when they chase you, pick them off with arrows in the water - that's how you truly want to use them. Early arrow firing makes it a piece of cake for them to regen, you want your range when they are low and far away since they have time to regen and killing them before it happens is best, not when you don't have enough time/arrows and they can regen it easily + they know you are there.
6: Random stuff - A few other random tips:
While being chased, climb up a ladder. Put a fire at the top of it. Anyone who is smart will not follow you up, since you can easily knock them down, and upon trying to follow you up, knocking them down will be a piece of cake since they will be struggling to put out that fire, or they will walk right through it for 4 hearts of damage.
While running from someone, traps and ladders/vines aren't the only way to get away - try finding a point on the map where there is a hard to make jump that gives you a huge boost, or some parkour. As long as you make the jump and he doesn't, he could die/fall far behind you and makes it easy to lose him.
You can spam right and left click when fighting someone to do a thing called "block hitting". Essentially, the idea is that you will hit them for normal damage, but they will only hit you for half damage. Personally, I don't do this, because whenever I try, the only thing that happens is that I don't hit them at all (because I do it wrong) and because for some reason people hate you for it (probably because they can't do it like me.

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When you are on the shore, and someone is trying to get to you in the water, fight back by sprint jumping in for a crit/surprise. Even if they were expecting it, it's still extremely tough to hit someone who is moving so fast around and over you, then under you all at the same time...for some reason, I find this works on most people who use forcefield too, which is pretty strange. Maybe it has to do with lag?
Oh, and I'll leave with a pro tip: Attempting to be a 'boss' and be better than everyone without knowing what you are doing by hacking is stupid. Just sayin'.