Using comets at Mars L1 point, allowing the tail to constantly add to the atmosphere in both volatiles and increased temperature from the infalling matter, as well as blocking the solar wind which comets are known to slow and divert. Then taking small 1 to 5 meter chunks (or larger) of the comets and dropping multiple a day in the Hellas Planitia, (lowest point of Mars), starting at the very lowest point there is a 10 km crater in the north central part of Hellas Planitia. Eventially creating lakes, capping the 10 km crater for colonization, dropping more comet rocks on Hellas Planitia (softer landings not hard).
The hope is to create a moderately tolerable climate in the 10 km (eventually) capped crater, 0.4 atm. And to create a micro climate in the lower elevation of the upper and western region of Hellas Planitia and eventually all of HP to grow foods, etc. (HP is very low compared to the surrounding terrian, and we may need to seal off some escape routes for the atmosphere to reduce strong wind tunnels forming for the higher pressure, though I do expect constant loss I want to minimize it.) Humans would walk freely under the eventually capped crater, but need oxygen and radiation protection in the HP areas (after the pressure has risen enough.) What issues do you see with this from a science point of view?