Hi mate congrats on the 55 , I had a go a few days ago and got to round 78 very easily . I got to 70 with no downs and left it on while I went to bed (it was like 3am) I carried on in the morning and it took some adjusting, felt very hectic because i'd come back on that round I started getting my downs then . It slowly went back to normal and I was training them fine then I saw a max ammo and lost concentration, was messing around shooting my thunder gun before I picked up the max ammo and sure as sh*% as soon as I was about to reload i got cornered like a idiot! could of easily got 100+ if I stayed focused.
My strat was I upgraded the servent as soon as possible , had the Zeus cannon and upgraded drakon with thunderwall . I stayed at the starting area next to quick revive until round 70 I just kept using the jump pad over to shangri la and up the stairs to get the keeper then back to the start. When I came back online at round 70 it felt hard so I went to kino on the stage and did trains there while grabbing the keeper which is close by.
I have a video of doing loops on 78 showing you how to do it and how easy it is ,also you get to see the shocking disgusting end to my game ha
Good luck on your future games