Niantic announced that Pokemon GO's first 4th Gen Legendary Pokemon Giratina is coming back to Raid Battles on 28 March 2019. Giratina will be in its Altered Forme until 02 April, followed by almost a full month in its Altered Forme.
"Giratina has reemerged from the Distortion World to renew and transform", Niantic wrote. You may recall that Renegade Pokemon Giratina debuted in Pokemon GO Raid Battles around Halloween 2018, albeit its appearance was limited to its Altered Forme.
This time around, Pokemon GO players are getting the whole deal when Giratina lands on Thursday, 28 March 2019, at 1 pm PDT (GMT -7). It will remain in Altered Forme until 02 April 2019 at 1 pm PDT (GMT -7).
Once this warmup is done, Giratina will transform into its Origin Forme and will remain in Raid Battles until Monday, 29 April 2019, at 1 pm PDT (GMT -7).
There won't be much need to change tactics from when Giratina last visited Pokemon GO, as Ghost/Dragon Pokemon are best served some of their own medicine, seeing as how they're most vulnerable to their own types.
Basically, Dragon, Dark, Ghost and Ice-type Pokemon should be best suited for the task, so think along the lines of Rayquaza, Dragonite, Palkia, Gengar, Mewtwo and similar.
Of course, Origin and Altered Forme swap their attack and defense stats, so although Giratina will be hitting like a hammer in its Origin Forme, you'll be able to defeat it quicker.
Ultimately, you shouldn't have much trouble with Giratina, regardless of the Forme, provided you do some clever planning beforehand. With six days to go until Pokemon GO sees the return of its Halloween headliner, you should have enough time to get ready
Note that Niantic's equinox celebrations are still on, which means that Grass-type Pokemon like Oddish, Exeggcute, Sunkern and Shroomish will be appearing more frequently in the wild. Grass-type Pokemon will also be available in Raid Battles until 26 March 2019, at 1 pm PDT (GMT -7).
You can find Niantic's announcement of Giratina on the official website here.