The recommended level is 50 but even with a full party around that level they still get torn apart because the gunners get a ton of hits that make chains before most units can approach and those fast enough to do it aren't tanky enough to survive the focused firepower more than 1 turn. This map is just designed to tear you a new one if most of your units aren't strong and enough to 1shot the gunners on the first turn.
Its difficult to answer you as we don't know your roster.
But bring Mont (or better) as Tank (with Vow of Love if you have it) + 1 healer + a friend unit who can carry you (Kitone is nice with high speed, shiruken)
You can fill slots with some range attackers (mages, archers or gunners)
Your units start in the middle, so choose a side, regroup, and let Kitone do her stuff