Use 63% 37% solder! Use <= 250C for <= 10s. 260C maximum.
USE A LOT OF, GOOD QUALITY, SOLDERING PASTE. This heats up the soldering point only. With such, you must be able to solder even the case pin.
For the pin which is connected to the case, 275C may be OK for a shorter period. Do not go over 300C. In case you go 300C, use < 5s.
AFTER EACH ATEMPT, BLOW AT THE CASE AS WELL AS THE SOLDERING POINT. BLOW A LOT. THEN, WAIT, THEN, SOLDER AGAIN.
EVEN AFTER A SINGLE, LOW TEMPERATURE, SOLDERING, BLOW A LOT ALL SIDES OF THE COMPONENT AND WAIT.
ADDITIONAL INFORMATION :
In regards to Flux and Solder Paste : Yes, some people use the term " Solder Paste " to denote powder solder, mixed with EXTERNAL flux. They, also, use the term " Flux " to denote any flux, the internal flux, which, is built in the solder wire and the external flux, which, comes separately and is in the FORM of a paste. Paste is anything which is semi liquid and semi solid.
The problem with the term " flux " is the ambiguity : is this the internal or the external flux. Maybe, the term " flux paste " would be appropriate, yet, I would use the term " External Flux ".
So, yes, I meant EXTERNAL flux, when I said solder paste in the original post, yet, solder paste may be even better as long as the quality is good.
In terms of external flux, the most important feature is, as mentioned, to start to boil and heat up, mainly, the areas of the components to be soldered ( covered by solder ) and around. Also, the external flux keeps the molten solder at the soldered pin and track only ( mainly ).
Thus, to solder a case pin with 250C is possible, yet, slightly more difficult. Here, again, is how :
Apply external flux to the iron only. Clean the iron with the wet sponge. This is for cleaning only.
Apply a lot of external flux to the pin and the track. Ensure all sides are covered.
Apply as much solder to the iron as possible to hold. Do not apply a lot of external flux to the iron in point 1., so the solder does not drip off of the iron.
Apply external flux to the solder wire.
Touch the solder point with the iron. Touch the so position iron tip and solder point with the solder wire, covered with external flux. Ensure the external flux starts to boil and the molten solder starts to spill out and cover the soldering areas.
Do not keep the soldering iron to the pin for more than 10s.
After the iron is removed, blow VERY LIGHTLY at the solder point to solidify the hot, still, molten solder. This happens in for a second or two. Then, blow as strongly as possible towards the soldering point. Then, blow as strongly as possible towards the case for a long while. Many blows are necessary.
Wait until the the case is at around room temperature. Touch the case with fingers to check. Solder another component while waiting.
Repeat all steps for the other side of the soldered point. Ensure the solders, covering, the old and the new solder sides become liquids and spill into one another. May need to touch the old side again. Again : do not keep the iron on the component for more than 10s.
Alternatively, 300C will do the job for sure. External solder helps a lot and not such huge amount is necessary.