Silly, but I keep getting stuck early on.
I've beaten the skill swordsman, but remaining three one the map kill me without much problem.
I'm talking about 3 slimes, ice golem and the overgrown hamster of tentacle forest.
I can do some damage to the golem, but it's just a sliver of his HP.
Slimes kill me it one hit.
With the hamster, things go reasonably well (...sort of), till it uses its earth skill.
I've got 祝福されし赤銅の長槍 as the weapon - it looks like the best thing currently available.
Which combination of equipment and skills am I missing ?
Since you're getting stuck on the "overgrown hamster", you probably don't have the 3rd-tier Crests available yet, but you seem to have finished the Bronze Mine, or at least with the capacity to finish it.
From my personal assessment, said "overgrown hamster" is the next thing you should tackle because 3rd-tier Crests, which provides a significant stat spike over the 2nd-tier ones.
The slime monster trio should probably be set aside until you have a full set of 3rd-tier Crests. The Ice Golem for until you can comfortably defeat the mooks in the undersea cave (none of them as terrible as the beefgate in the front, thankfully, aside from the boss that is), or preferably after defeating the boss of the undersea cave, with a couple pieces of Iron gear.
Take an assessment of the gear you're currently equipping. Your gear should be mostly cupronickel tier(白銅). If you don't have it yet, finish the Bronze Mine, the one with all the zombies. Since you're having trouble surviving, you can consider equipping Earth crests or a Staff/Mace piece of equipment for the buff skill Clay Armor (クレイアーマー). The staff/mace also provides some decent healing, if you don't use the Water crests anyway. Also - and I think this is probably more important - you may want to invest in a suit of Cupronickel heavy armor, and/or a Cupronickel greatshield. Especially the greatshield, they have the skill Guard Stance, which increases DEF by 50% for one turn. That should reduce the impact of Mr. Big Guy's ultimate attack to relatively reasonable levels.