Koronium is a dangerous Metal enemy. Don't let his 150 health fool you, as Koronium, just like his White and Red variants, is able to create shockwaves with every attack (level 4). Koronium has more range and attack power than his variants.
Critical Attackers are almost mandatory to turn this eccentric koala into scrap metal. DO NOT USE ANYTHING WITH A RANGE LESS THAN 240 (except perhaps wave blockers). The use of wave blockers is also recommended to try to blot out his wave attacks. However, beware as this koala has ridiculous amounts of DPS that can shred your wave blockers. Long ranged wave blockers like Hatsune Miku: MM2020 Tokyo, Eva Unit-00 and Mighty Kristul Muu (luring is needed with Muu) can easily counter it though. He hits considerably hard up close, so getting past his range buffer of 240 by using ranged Critical Attackers such as Curry Cat, Space Marine Cat or Cameraman Cat with Talents is worth it. Curry Cat works VERY WELL with his slow ability and Koronium's slow movement speed. Talented Lollycat is also very good against it, though beware that it requires a lot of NP. If you have E. Kubilan Pasalan this enemy will be a joke because not only can he one shot him but his waves can reach him from a far away distance.