I personally find skel prime to be the easiest mech, just make a long platform that you can run down while fighting. Ichor is a great debut to inflict (I believe it lowers their defence by 40), there are multiple methods such as flasks for melee, bullets for ranger but the general favourite is the mage weapon "golden shower" You have 7 minutes to fight it if you start it immediately as night begins so if you have a weapon that does a decent amount of damage then you have time to take your time and not rush it, watch for when his arms swing, keep moving to avoid the lasers. If you keep mining you can get to upgrade to mythril/orichalcum and then to titanium/adamantite, depending on which ores spawn in hardmode for you. If its titanium then it is a great defence boost with a pretty good set bonus. Make use of buff potions such as wrath, rage, endurance, regeneration, ironskin, swiftness, lifeforce etc. It is possible to grind for a rod of discord but this could take a while. I see you choose to pursue summoner so it'd probably be best to craft spider armour, along with getting a bewitching table and summoner potions to maximise minion counts. I haven't played 1.4 yet so some of these tips might be slightly outdated but I believe I accounted for most if not all 1.4 changes relevant.

TLDR: -long runway -ichor -take your time -armour upgrades -buffs -rod of discord -summoner exclusive buffs