I have a Note 3. It came out a little earlier (2013 vs. 2014 for the S5) than the Galaxy S5 so they maybe similar in performance...
I just soloed my 1st, a Machamp. It had Bullet Punch/Close Combat...Prob easiest movesets are 1 bar charge moves since you KNOW when he will launch it when you are solo.
I haven't soloed others yet, but that could also be due to not trying (not wanting to waste passes).
I have only tried to solo Arcanine 3 times...My best failed team has been very close with hardly any red health left...Maybe less lag and I could win, but don't really have time to keep trying over and over again...I pretty much have unlimited potions/revives so it's mostly a time thing for me...
I think lag plays a bigger role than people make it out to be since if your graphics, animation, everything is 20% slower, you will simply also do 20% less DPS. I think this is because when my wife does her evolve parties, she can easily do 90+ (iPhone 7Plus) and I normally only get 65 or so...
Try Machamp or I hear Gengar first...Focus on easier boss movesets with 1 bar moves...Dodge at least 1 and make sure everyone on your team unleases 2 or 3 Future Sights...That's pretty much it I believe...