Then probably Ness, all thanks of paralysis.
Paralysis mayor traits are the fact that:
1- It reduces mobility by a big margin. This can be seen in the description of the abilitie, which states that it numbs the body to a degree that dificulties proper movement. In game, when a character gets this status, they can be seen in the overworld barely moving thanks to it.
2- It completly nullifies physical combat. When hit by it, the abilitie to use weapons for face to face combat is destroyed, this can be seen when everyone that gets it by it can't no longer fight with melee attacks and the only way to counter attack is by PSI, attacks that can be only be done with the use of the mind. It also blocks range weapons, as seen as it block people like Jeff from using his guns and bazookas. Basically, everything related to physical weapons or attacks is gone.
3- It blocks the use of equipment. This is probably the biggest treath to Terrarian. When it by hit, characters can no longer access their items to use during the combat. This blocks Jeff from using his rockets, missiles, bombs, and to everyone else, healing items, stats buff and debuff items and even different weapons. It even block them from changing weapons and armor mid fight.
4- Is a status effect that doesn't disappear by time. It will stick until the end of the battle, and whitout healing items or PSI skills, it can't be cured.
So basically, I see the fight going like this:
They start fighting, either Terrarian cast his buffs to gain advantage or faces Ness instantly attacking and trying to debuff him, and Ness tries to either use Hypnosis and Paralysis. Why use supportive abilities before stuff like Rockin?, becuse Ness seems like a supportive character by nature, literaly, besides Rockin and Flash (Which its only damaging property is its OHKO chance) everything else he has for his PSI abilities are supportive, and not wanting to use debuffs to enchance your chances of victory seems dumb. If Hypnosis is use, it will probrably fail, since Terrarian is an expert in crafting futuristic weapons and advance technology, wich also means he as a big IQ, wich is similar to Jeff's, so that won't work, so he'll resort to Paralysis.
Since i'm not seeing resistance to it in his page, i'll asume it will work. If he starts with Paralysis ignore the above. Once hit by it, Terrarian will lose a big portion of his hax, since as stated above, everything he has comes from items, which are blocked once hit by paralysis.
His mobility will also suffer, since as also shown in game, enemies like birds and even UFO's suffer the consequences of paralysis, so Terrarians Wings or gadgets will become either useless or inefficient.
His combat will also become useless, since he will not be able to use his swords, guns, magic books, staffs or everything for that matter.
Since sutff like his healing potions and big hax are on his inventory, he won't be able to access to them, so he only would have access to what he has at the start of the fight.
And considering the fact that paralysis doesn't worn out, he's stuck like this for the rest of the fight.
Next act is Ness either hiting him until he falls or casting Rockin omega.
Regarding other aspects such as experience, I think they are evenly matched, since both of them fight some weird sh**t during their adventures, and I highly doubt survival and construction stuff would work during battle unless there's prep time, and even if Terrarian tries to construct something during the battle, he'll be unable to reach his construction materials thanks to paralysis.
so yeah ness.