It should be both Lakainu and Lizaru. But both can survive, so SH Pirates can beat them too.
As for Lakainu can't go anywhere excuse, Blackbeard pirates can come to them since they are already in the new world, and also previously the marine headquarters weren't in new world, thus it was harder to leave the base, now the base is in the new world, so the Fleet Admiral can travel if its necessary in the new world, it's not a long distance to travel in comparison with before.
Lakainu and Lizaru were portrayed as duo several times, they even joined the Larines in the same year.
If Blackbeard two-shots Lakainu and doesn't get heavy damage in return while two-shotting Lakainu, that will both show the correlation between Whitebeard and Blackbeard, and it will show that Blackbeard is far superior to nerfed marineford Whitebeard, meaning that he is on the same Whitebeard's level or even superior to Whitebeard. There isn't a better way to show how Blackbeard surpasses Whitebeard.
Also Shiryu too should beat Lizaru to prove he is a legit top tier, Shiryu needs to do something big to catch-up on the other Level 6 prisoners bounties. Ivankov mentioned the names of Level 6 crew of Blackbeard pirates before even they escaped from Impel Down, meaning they already had large bounties before joining Blackbeard pirates, while Shiryu started as 0 bounty just like Blackbeard, Blackbeard obviously surpassed Level 6 bounties, due to being the Captain and a Yonko, and the leader of his crew, but Shiryu might not surpass other Level 6 crew without doing something big, something that can separate him from other crew members.