If you want quick levels, the best approach is to just capture the highest level rare Morty that you can. They will be relatively weak compared to a well trained Morty though. Same using level-seeds.
If you have a second account/device, you can just keep battling and gaining levels that way, between the two accounts. This will be stronger than the first approach I gave, but still relatively weak.
If you take your time and build your Mortys, ensuring that you at least daze as many wild Mortys as your current level, before gaining a level, you'll find a decent compromise in leveling speed and relative strength.
If you maximize how many Mortys you daze before leveling (usually done by fighting lower level wild Mortys), then you'll get the best return on your stat increases as you level. There's another thread regarding maximizing EP so I won't go into that.
Well then get your shit together, get it all together and put it in a back pack, all your shit, so it's together. And if you gotta take it some where, take it somewhere, you know...