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About epic and worthwhile boss fights, there are many things to consider. Let me give you some tips, bro:
We all know Bosses are unique and so you must make them.
First, their stats. You must make their stats ftting for a real boss or monster like. So it will be like big Life and Mana. It doesnt need to be too big if t has strong armor and attack damage for it will make it too imbalance. So two options here are either: strong Life and Mana; decent attack and armor or decent Life and Mana; strong attack and armor.
Now we are finish with setting up the bosses stats, now let us ensure that the environment we are gonna put the boss is fitting. What do I mean? What I mean is that the boss should be in a place that bestly fits him. You cannot put a Water Elemental Boss in a Volcanic Area right? So put him at the right place and voila.
Mostly we can all know, map makers kind of rely on the Bosses' might on defeating heroes but to add more difficulty. Maybe you can add more active traps around the Boss Area. This will make it harder and grant advantage to the boss. Maybe it be traps that damages, gives a negative buff or does both.
Now let us go on the to the boss itself. If you want to make a cool boss, make sure it has cool spells. Original Warcraft spells just looks bad and unfitting for them. So best is fit it with cool, special and unique spells still making sure it fits woth their style and theme.
Boss battle will be more enjpyable if it is done phase by phase. What do I mean? Some bosses when reaching a certain percentage of their life, they either cast a strong spell to harm a group of heroes or summon minions to help them fight. You can do this to add some more fun in the game.
Now last is the reward for the effort of killing a boss. You must ensure that the reward you gave for killing a boss is just, enough and worth. You dont want to get a Branch that gives 1 stats by killing a hard boss, it makes the gameplay so worthless and such so last tip is give bosses an enough, just and worth rewards or drops upon defeating them.
Well that is my perpective of making a good Boss fight. Hope this helps you in many ways.
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Straight ill man, this sounds like it's a quote from some sticky thread
should be followed by many RPG makers, i say