Mmmhh, I don't know...I have my reservations, personally. First, Mercury is retrograde, I don't know if this is a good idea...but you can see if an astrologer answers to this.
What Mercury rules over in people and on the planet. Themes of Mercury
So, Mercury rules (on a macro level) everything related to communication (whether that may be in written form, like contracts, texts, via computer, journalism, writing, books, written words) or even means of transport in a broad sense (trains, plains, boats, ships, cars ecc...).
So, communication seems to be key for this planet. In fact...on a micro level, Mercury rules your throat chakra (ruling over communication skills, the ability to communicate efficiently, expressing yourself, ecc..., also the nervous system, which receives and gives messages of external or internal stimuli).
That's not the only thing, though.
Mercury is associated to intellect as well: intellectual interests, what kindles your intellectual interest, education (as in macro, Mercury rules over books, means of education, you would say "the communication of information" in a broad sense).
In micro, Mercury is also associated to the brain and wittiness, your "way with words", again, efficient communication done with intelligence (maybe even strategy), with correct language and general communication skills (speaking at the right time and/or in the right way to be taken seriously).
The throat chakra is also connected to emotions, particularly...the correct expression of emotions, again...communication and the way you convey how you feel. I had a blockage in this chakra because I couldn't properly express my emotions due to fear. So, I would say, Mercury also rules over how you express your emotions, communication of emotional state.
Comparison with Mercury the God
Remember that Mercury in the Roman Pantheon is the Messenger of the Gods, also known for eloquence (effective communication) and persuasion. That is a huge clue to understand what Mercury (Planet) is about.
Safe methods for concentration and focus
So...if your goal is to focus on tasks and concentrate...I don't know if working on Mercury is right for you.
I would personally use such a square for:
1) Communication skills
2) Developing witness and way with words
3) Sharpen intellect
etc...
Because if you increase Mercury's influence in your life...you're also working on the throat chakra, indirectly. I mean, Mercury affects the brain, and maybe the ability to process information, but I think you want to do something specific for grounding and focus.
Void meditation to increase attention span (especially the "EVERYDAY VOID MEDITATION", excercising your attention span by forcing yourself bit by bit to concentrate on a task. Do grounding exercises, staying in the present, working on the Root Chakra can help.
Warning: study the theory extensively before practice
I think you don't want to dabble in something you don't 100% know you need, especially if the theory about your practice is not clear enough.
I would do a Mercury square only if I 100% understand what the planet rules, how it affects me and what is around me. You want to use it if you really need it: if you have a debalanced Mercury (have trouble communicating in a good way, expressing yourself emotionally or intellectually, intellectual processing ecc...). Planets rule over more than one thing in one's life, so if you do that, be prepared to have the desired effect on one thing, but it will also affect other things (with how I see it).
Planetarian Influence is complex and broad on person and the world at large...so, be aware you know what you are doing.
If your problem is just focus, that's more specific, and you don't necessarily need a Square you don't know the full effects of...on you or the Mercurial aspects on your AstroChart or Life.
There are other ways that are safer and go straight to the point (what I had just discussed).
Hope this is helpful...