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The positioning of both your party members and your enemies has a pronounced effect on the outcome of any given battle.
When fighting against enemies with powerful ranged attacks, it is especially important not to let your entire party get clumped together in one spot--a well-played attack could wipe you all out at once.
Just like you, your enemies can use extended attacks, and can concentrate their attacks on particular members of your party. If you don't plan well, you could be defeated in the blink of an eye.
As such, it is important to understand the roles of each one of your comrades, and position them in battle accordingly.
Keep this in mind when distributing ability points after leveling up. You might want to focus on strength for a particular character, for instance, so that their attacks hit harder. Focusing on agility for another character will mean they--and consequently, any party members connected to them--can act earlier in each encounter. A character with high HP and good defense will be able to take more damage, and protect weaker members of your team. It's also a good idea to keep weaker characters in back, where they can heal other party members.
If you pay attention, you'll eventually be able to determine the attack patterns of various different foes.
Once you can see these patterns, you'll have a much better idea of where to place your characters to keep them alive longer and win the fight.
If you notice that your enemies are linked, make sure to sever the link as soon as possible to prevent extended attacks. Generally, the best way to do this is to attack and defeat the central character in the link formation. If you don't have the power to defeat them quickly, status ailments like paralysis or skills that can forcibly move the character they attack will serve the same purpose.
Success in battle also depends on taking advantage of the specific characteristics of your party members, your opponents, and the field itself. This might mean defeating characters with lots of AP left, using extended attacks to move weaker characters out of danger, or a multitude of other things.
There's no need to panic. Certain items will let you recover, and extended areas will even let you heal people who aren't next to you.
Keep in mind that if one of your characters is defeated, everyone else on your team will get a surge of SP, allowing them to quickly unleash powerful attacks. Cold though it may be, learning to use this to your advantage can mean the different between victory and ultimate defeat.
Of course, you don't want your characters dying all the time, so a good rule of thumb to keep in mind is that Borgnine is to be used as a wall that absorbs attacks, and Ellis is to be protected.