There are different method that are also effective when saving up Bitcoin as investment but in all I believe DCA method is practically the easier and more cheaper because you can start with whatever you have as set it as a consistent value to which you buy every week, twice monthly or once depending on your financial stability and at end HODL up the investment for long term as it's bound to yeild good profits overtime.
I wouldn't say it's the cheapest. It's not like other strategies require anyone to invest more than you would by doing the DCA. The amounts you're willing to invest and the strategy you choose are two, separate things.
In terms of being the easiest. I think the easiest strategy is just to make a one-time purchase and just hold forever, it doesn't get any easier than that. But the DCA is probably the most convenient and easiest to execute among all the active strategies.
The trickiest way is to know when to cash out (if ever).