In Ibn Taymiyyah Al-Sarim al-Maslool book, pages 582-583 (archive.org) there is a hadith narrated by Ali from Muhammad (saw) saying a person is promised paradise for anyone who kills Rafida (Shia). Is this hadith authentic?
Is a hadith in Ibn Taymiyyah about whoever kills Rafidhas being promised paradise authentic?
No. At least it won't be authentic if its purpose is Shia Muslims. It is not logical in Islam to kill several Muslims to go to paradise! And when it is a common issue that there can be some/many authentic hadiths (traditions) in diverse books of hadith, hence we ought to be fair/clever not to accept the hadiths which seem to be in a paradox with authentic/famous ones.
Such hadiths easily can be rejected by investigating in authentic/famous hadiths which demonstrates the high position/respect about Shia Muslims. Consequently, by watching such contradiction, you can perceive that there seems to be a problem in its (their) authenticity.
Let's investigate regarding high position + respect of Shia Muslims, and ponder if it is logical to kill them to go to the heaven? (By fair judging):
اما انك يابن ابي طالب و شيعتك في الجنه
“O Ibn Abi Talib, indeed you and your shi'ah shall be in paradise.” Following this hadith is a statement against the rafidis attributed to the Prophet (S).
Al-Dhahabi Mizan al-i'tiddl, vol 2, p. 18. and: https://www.al-islam.org/al-tawhid/general-al-tawhid/shiism-and-its-types-during-early-centuries-part-1-rasul-jafariyan-0
Or for instance, pay heed to Hadith al-Thaqalayn as a famous authentic hadith from both Sunni-Shia view. (Note: Hadith of thaqalayn is narrated a little differently in different sources, but the content is the same):
Al-Kafi which is one of the Four Books of Shi'a:
إِنِّی تَارِک فِیکمْ أَمْرَینِ إِنْ أَخَذْتُمْ بِهِمَا لَنْ تَضِلُّوا- کتَابَ اللَّهِ عَزَّ وَ جَلَّ وَ أَهْلَ بَیتِی عِتْرَتِی أَیهَا النَّاسُ اسْمَعُوا وَ قَدْ بَلَّغْتُ إِنَّکمْ سَتَرِدُونَ عَلَی الْحَوْضَ فَأَسْأَلُکمْ عَمَّا فَعَلْتُمْ فِی الثَّقَلَینِ وَ الثَّقَلَانِ کتَابُ اللَّهِ جَلَّ ذِکرُهُ وَ أَهْلُ بَیتِی
"Indeed I am leaving two things among you, to which if you hold yourself, you will never astray: the book of Allah –who is all mighty and great- and my ahl al-bayt (household), my 'itra (family). O people hear! And I have announced to you that: indeed you will enter my presence and I will ask you about what you did to the thaqalayn (two weighty things) and the thaqalayn are the book of Allah and my ahl al-bayt."
Sunan al-Nasa'i, one of the six sahih (authentic) books of Sunnis:
کأنی قد دعیت فاجبت، انی قد ترکت فیکم الثقلین احدهما اکبر من الآخر، کتاب الله و عترتی اهل بیتی، فانظروا کیف تخلفونی فیهما، فانهما لن یفترقا حتی یردا علی الحوض
"Looks like I have been called, and I answered (my time of death has come), indeed I have left the thaqalayn (two weighty things) among you, one of them is greater form the other, the book of Allah and my 'itra (family), my ahl al-bayt (household). So look after how you will behave with them after me, indeed they will never separate each other until they enter my presence by the pool [in the paradise]."
How can we accept such hadith (a person is promised paradise for anyone who kills "Rafida (Shia)) by noting/comparing to authentic/famous hadiths which show/prove the high position of Shia and its followers?
Meanwhile, recognizing who is the real Rafidi is another significant point. Hence, it also can be wrong/distorted about the definition of Rafidi (which needs a long discussion).
The only hadith (saying of the Prophet () himself) which ibn Taymiyyah has quoted -it is on page 582- on the site you provided which says so is from Musnad Ahmad, and ibn Taymiyyah confirms that it was added to the Musnad by Ahmad's son Abdurrahman. But as we know that the son of Imam Ahmad (see my answer here Is Musnad Ahmed a book of Sahih Ahaadith?) added many weak or fabricated narrations to his father’s Musnad we can say that this hadith has good chances to be one of them!
Note: ibn Taymiyyah didn't even quote the full chains of his 2 pages of narrations so it is hard to source them. Interestingly all the narrations are from Ali ibn abi Talib except a general statement about the hatered of ahl-al-Bayt by Talaha!