This achievement is possible on solo and here are some good tips.
To get this achievement you must hit wave 25(2) (which is basically wave 50) every time the game hits a new "flip" it will say 1(3) or 1(4) the (0) in () is the current # flip you're on.
First off use the lightweight class (perfect class due to exo-dodging from side to side)
you can dodge with your exoskeleton indoors (but press
once and then direction of your choice to dodge in.
Now as for my tips the easiest map I have found so far would be
Detroit (Teir 1)
Instinct (Teir 2)
Solar (teir 3)
The EASIEST MAP would be Instinct, there are a lot of areas to dodge and avoid enemies.
Now, the main goal is you'll just want to camp in the very back caves (at the back of the map) and just try to stay in there. (run out if too many are coming and start doing trains around the map (similar to nazi zombies) don't let the enimies build up though, just avoid them and let them chase you where its easier to take them down.
This strategy is flawless and I myselff have gotten to wave 88 offline (but still got the achievement) when I came online.
Now as for tips.
Max out your armor first (armor will keep you alive longer) the highest level to max out your gear is level 10
max out your armor and then your weapon. When you max out your weapon (your weapon will do x2 - x3 more damage)
now last but not least your weapons (the most important part of the fight)
KP5 is a piece of garbage after round 5, so upgrade to the BAL-25, and then when round 10 comes have your second gun exchanged for the HBRA3.
You'll want to upgrade your armor first and then your weapon second.
Another very important tip, when the drops come keep getting UAV's and Goliaths.
During difficult waves (you think you might die in) activate your Goliath, if you die in the Goliath you wont go down, you'll still be alive (you'll just respawn in a different area) Just think of it as an extra life.
but don't constantly keep calling in your killstreaks (save up all four of them) and then let the drops keep dropping perks (fill up all of your perks until you can't carry any more) and then call in your killstreaks.
This strategy is by far flawless and can easily get you the achievement. It will take around an hour or so to do on solo, but at least you wont have to worry about lag or teammates constantly going down.
(Sorry for this being so long, but I really hope this helps.)