Minions are not for having level 30 goblins. The highest-level minions in the 4e MM1 are the Lich Vestige (level 26), Abyssal Ghoul Myrmidon (23), Grimlock Follower (22), and the Angel of Valor/Legion Devil legionnaires (both 21).As a result, you had creatures that were 30th level goblins, which would decimate any party below level 25 because of the power of their attack and defenses, but would still die in one blow. Where is the logic here ? What are these creatures, exactly, where do they come from, how do they relate to the general population of goblins ? No-one knows, they are just pawns to be pushed on a grid to occupy the adventurers for a few rounds.
- Lich Vestiges are remnants of destroyed liches. They still channel immense arcane power, but are very fragile. They're the kind of thing you'd find floating around Orcus.
- Abyssal Ghoul Myrmidons are the minionified version of the level 16 Abyssal Ghouls.
- Grimlock Followers... yeah, I got nothin. They're basically filler for powerful underdark encounters.
- Legion devils are fiendish footsoldiers that exist in various ranks from 6 to 21, meant to fill out encounters with devils or devil-aligned opponents. Angels of Valor fill the same role for divine servants (notably, in 4e angels are more mercenary, and can work with/for any god, so e.g. Bane probably has legions of angels at his disposal just like Bahamut does).