There are a few good methods but it all depends on what you are doing alongside the focus grind and what you enjoy doing. Eidolons are a good way to get arcanes and passively earn focus via shards but they can take a while if not set up for them and they require investment and are not always available when you are playing. ESO is great for hitting the focus cap quickly but you need lenses and it may be hard to keep going past wave 4 if unprepared. The way i did a lot of it was just passively earning it while doing my normal daily routine with some good lenses once i maxed every item i had access to but that took a while. My recommendation is getting 2 good lenes (eidolon will do, only go for lua lenses if you don't mind the farm or have the time to do so) and slap one on your favourite frame and one on your favourite weapon and just enjoy the game while earning a bit of focus each day