“We are going to do something that I will say is slightly controversial but it shouldn’t be. We are going to indemnify policemen and precincts and states and cities from being sued. We want them to do their job. Our police and law enforcement has to come back and they want to come back and …
What about if instead of giving police a free reign to commit brutality, he said something like "we're going to vastly increase the funding that goes towards quality training; we're going to reduce reliance on old military weapons and tactics; there will be less lawsuits because our police will be given a quality education."
How about wanting the police to be the best at their jobs, rather than the most brutal?
I'm sure a lot of potential cases would actually be filed in state courts where he would not have much power over, at least directly.
As far as civil vs. criminal, ianal, but
Under traditional English common laws, a doctrine called sovereign immunity prevented citizens from suing the king. Sovereign immunity was incorporated into American law as well, imposing limits on lawsuits against government agencies or employees.
source: Forbes.com
So basically, the government has to grant you permission to sue it. If a president decrees that the government is immune in a civil case, there is little legal recourse. (Reminder: ianal, I want everyone to know ianal.)
But the big picture: regardless of whether efforts to interfere could be successful, this announcement will increase police brutality.