On the 27th September 1942, the Liberty Ship SS Stephen Hopkins, carrying cargo from Cape Town to Suriname, encountered a German surface raider. This raider, the Stier, had been cruising from the 19th of May. She had encountered three Allied merchants, sinking all. She considerably outgunned Stephen Hopkins, being armed with six 5.9in guns and several lighter weapons, compared to Stephen Hopkins' single 4in gun. However, due to foggy weather, the action opened at close range. The crew of Stephen Hopkins were able to use this to their advantage, scoring several accurate hits. They damaged Stier's engines and steering equipment. However, Stier's heavier fire would ultimately tell, reducing Stephen Hopkins to a drifting wreck that would sink an hour after the end of the engagement. Even so, Stier had been very heavily damaged, causing her captain to scuttle her.
Also worthy of note is the SS Ondina, a Dutch armed tanker. On the 11th November 1942, she was travelling from Australia to the port of Abadan in Iran. Ondina was escorted by the Bathurst-class corvette HMIS Bengal, though the warship was actually less well armed. Bengal was armed with a 3in 12pdr gun, while Ondina was armed with a 4in gun. The two ships encountered two Japanese raiders, Aikoku Maru and Hokoku Maru. These two sister ships (converted from civilian liners) were armed with 8 5.5in guns each, as well as four torpedo tubes. When the ships were sighted, Bengal's captain turned to engage, thinking Ondina would disengage. However, Ondina's captain decided to engage as well, believing it gave him the better chance of surviving. One of Ondina's first hits was scored on Hokoku Maru's stern, where the torpedoes were stored. They exploded, causing devastating damage to the converted liner, and leading to her sinking an hour after the battle began. Bengal engaged Aikoku Maru, but was forced to disengage due to a lack of ammunition. With her out of the way, Aikoku Maru was free to engage Ondina, closing to point blank range, and forcing her crew to abandon ship. She fired two torpedoes into Ondina, and left the scene. Despite the heavy damage, Ondina didn't sink, with her crew able to reboard her, and bring her back to port at Fremantle, Australia.
The SS Stephen Hopkins has the distinction of being the only US merchant marine vessel to sink a German combatant afloat during World War II.
On September 27th, 1942 the Hopkins was sailing to Dutch Guiana on the last leg of her maiden voyage, and was jumped by the German commerce raider Stier and another armed German merchant ship, the Tannenfels. Just to give you a rough comparison in size, the Stier displaced about 11,000 tons and had a half-dozen 6" guns (and associated smaller armament), the Tannenfels roughly the same but much more lightly armed, and the SS Stephen Hopkins about 7200 tons, with a single 4" gun and some machine guns.
The Germans spotted the Hopkins at about two miles, and when she refused to surrender both German ships started pumping shots into her. Over the course of about 30 minutes, the Hopkins was shot well over 30 times and raked with machine gun fire, while her gun crew (and volunteers, after the original crew was killed) manned the 4" and fired back at the Stier, with machine gun nests raking the Tannenfels.
The end result was that both the Hopkins and Stier were flaming wrecks that sunk pretty quickly, with the Tannenfels picking up Stier's surviving crew (all but a had survived the firefight). The Hopkins didn't fare so well, being scuttled by surviving crew, who then took to a lifeboat and drifted for over a month before being rescued.