The U-8 can be a tough boss mostly because of its resilience and its very strong attacks. In order to defeat it, continuously move and fire at its mouth or the small, red-colored section of its legs. Firing at the mouth is advised because the area of the legs is small and while the U-8 moves, it is difficult to aim at them. When it closes its mouth, however, you have to fire at the legs. You can also try throwing a grenade into its mouth, which, if you are lucky, will cause significant damage. When it collapses because of gunfire, try throwing a game-given grenade in its mouth. Avoiding its attacks will grow more difficult, but if you constantly move in circles around the platform's pole you will avoid some hard hits, although it will follow you. His least dangerous attack is the launching of small infected flies which you can avoid by constantly moving and, whenever you can, shooting at the flies with a shotgun if you have one. Repeat this strategy until U-8 is finished.
If you run low on ammo, simply wait until the U-8 climbs up and releases the flies to attack you, since they die very easily and drop ammunition. Another thing to note is that the U-8 has somewhat poor hit detection and therefore it is advised to aim at the upper part of its exposed flesh, as the lower part of it will sometimes absorb bullet fire as though it were still protected.