Big Icedus (ビッグアイスダス), the boss at the end of Icecap Zone Act 1, is a small machine that looks a little like a gray snowman that summons eight ice balls to swirl around it. At the beginning it is, alas, out of hitting range (even when using the Insta-Shield) unless you have a lightning or water shield.
After it surrounds itself with ice balls, it swoops across the screen twice; stay in either corner and you'll be safe. Then stand below it and wait for it to release his ice balls. Then jump and hit it, preferably twice, before it moves up out of hitting range. Then comes the tricky part; five of his ice balls fall from the sky once, and you must slip in between them to avoid getting hit. They fall quickly and without warning so you must react quickly. After this it repeats the pattern, sometimes swooping across the screen once instead of twice. Keep hitting it after it releases its ice balls. Six hits does it in.
Aside from the variations to the attack pattern, this boss is very reminiscent of the Metropolis Zone boss from Sonic 2 in that it protects itself with a barrier made of several "spheres" (in this case: its ice balls) while trying to ram into the player.
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Sonic and/or Tails can get into Knuckles' boss arena by using the glitch with the vertical scrolling to go beyond one of those walls which can be destroyed only by Knuckles. Once the miniboss is defeated, the beginning of Act 2 will be using a weird palette (which will get eventually fixed by itself at a later point in the level).
Wow, there is a lot of light in this cave
It should be noted that the miniboss appears only in Sonic 3 & Knuckles, as its object hadn't been placed yet in plain Sonic the Hedgehog 3
; because of this, and since the boss arena is actually part of the Act 2 layout, Sonic and/or Tails can effectively use Knuckles' path to skip the miniboss fight altogether.