Superboy (vol 1) #234 VF

WANTED -- DEAD OR ALIVE, THE COMPOSITE LEGIONNAIRE
The Legion hunt down a space dragon whose radioactive scales help the people of Imsk maintain their shrinking powers.  When too many scales are removed at once, the dragon explodes and merges Shrinking Violet, Colossal Boy, Lightning Lad and Saturn Girl into a berserk composite Legionnaire.
While traveling to Imsk, Shrinking Violet's home planet, she informs them about the rare space dragons that live around Imsk.  The have rare radioactive deposits that allow Imskians to shrink. Colossal Boy tries to mine too many of the radioactive deposits at one time and causes four of the Legionnaires, Colossal Boy, Shrinking Violet, Saturn Girl and Lightning Lad, to amalgamate into a composite Legionnaire.  After the composite Legionnaire wreaks havoc upon the galaxy, a bounty is put out but the other Legion of Super-Heroes members try to heal their teammates before it comes to that.  Thanks to Wildfire, the composite Legionnaire is finally stopped and the original members are in their rightful forms, but seemingly at the sacrifice of Wildfire.