I have played fewer than 10 games of Papagra, so these points may not be valid in the long haul.
It seems difficult to get beyond the 6 to 7 matched-group size. If that is going to be the norm, I see an advantage in preparing for those groups at the expense of anything smaller. After all, if you get there first, it's yours for keeps.
Be careful not to setup the next-largest set of open spaces for a matched-group, since your opponent is likely to be able to complete the group with his own stones. This is especially true of the smaller groups.
I think strategies you employ depend on those your opponent uses. That may be too obvious to bother stating, but it's worth considering in this game as in many others. Take football, for example. If the other team takes away the run and the long pass, throw short routes to nickel and dime them to death. If your Papagra foe lets you build what you want, do it. If he interferes, use his stones to your advantage. Just don't rely completely on them always being there. Remember, they can be removed.
Try to setup multiple paths to your goal. Create situations where, no matter what your opponent does, it will help you. If he blocks your next move by putting his stone amongst yours, see if you can use it as part of a matched-group "ring" around some empty spaces. If he stays away and builds in his own area, try to setup (at least) three potential scoring areas where he can stop only two of them.
If the opportunity arises, use the removal of a stone to create half of a matched pair and the placement of a stone to finish it. (This feels like moving a stone from one circle to another.) Such a move is close to causing "check" in chess by moving a piece out of the way and letting another actually do the "checking".
That's all I've got for now. If I have time to play more before 2/21, I'll modify the above.