Well, I think the first question we should address is "Why do you want to use Guard Stance over Fury Stance?" If your idea is that it'll make you more tanky, then you are correct; it does indeed make you more tanky. However, your damage will plummet significantly, and by that I mean, you will might as well not do any damage at all. With that out of the way, I'll go over some basic math to show the underwhelming performance of choosing Guard Stance over Fury Stance. Before we start, keep in mind that all calculations henceforth will focus only on melee damage received (to you by enemies) and melee damage dealt (by you to enemies). Following what you suggested, the Hunter skill tree I have made for this case will have all skills regarding Guard Stance, all melee boosters, Iron Will and Stalwart Spirit at max. https://bit.ly/2UzlrLh
Let's start first with the Guard Stance focused Hunter skill tree. At max Guard Stance, we receive only 75% damage or in other words, we get a 25% damage reduction. Assuming you're going all out with Guard Stance, we will include Guard Stance Up as well for argument's sake. A max Guard Stance Up gives us another 90% damage taken or 10% damage reduction. Now, I will include Hunter Physique under the assumption that you want a tanky build. A max Hunter's Physique will give us superarmor as well as a 75% damage taken or 25% damage reduction. I will also just mention Flash Guard 1 as well since a minimum of 3SP is required in order for us to unlock Hunter's Physique. At 3SP, Flash Guard 1 will give us a 94% damage taken or 6% damage reduction. Below, I will show how damage redcution work in the game.
Damage reduction stacks mulitplicatively; in short, you get diminishing returns for each investment. Let's do some math to show this point. Keep in mind that the actual damage calculation is more complicated, and what I'm showing is just a simplified version of it -- for all intents and purposes, this simplified calculation will work [damage received] x [1 - damage reduction]. Let's say that an enemy deals 200 melee damage to you before any damage reduction is calculated. How would all the skills above reduce the damage? As mentioned above, damage reduction is multiplicative so here's how that math would look like.
200 [damage received] x 0.75 [Guard Stance] x 0.90 [Guard Stance Up] x 0.94 [Flash Guard 1] x 0.75 [Hunter Physique] = 95 total damage received. As we can see, with all the reductions above, you essentially get a total of 52.5% damage reduction [1 - (95/200)] from any melee damage attacks. Doesn't sound too bad, but now the real question is, is it worth sacrificing Fury Stance for this damage reduction?
I will now shift my focus from damage reduction to damage dealt by you. While providing damage reduction, without being a Hunter-main class, Guard Stance will essentially provide zero damage multipliers for you. This means that your damage while using Guard Stance will rely solely on your damage multipliers from your Fighter skill tree. Now, I will discuss the damage potential you do receive from the max melee boosters from the Hunter skill tree. Although Stalwart Spirit does provide a huge Mel-Atk stat boost of +300, due to its heavy reliance on RNG, I will not be discussing it. With that out of the way, let's get started.
The Mel-Atk you receive from all three melee boosters at max is 50 + 75 + 100 for a total of +225 Mel-Atk. While it sound like a huge boost, let's actually breakdown how much this translates to actual damage increase in percentage. For simplicity as well as due to currently low atk stats, let's say that +30 Mel-Atk translates to about 1% damage increase. This means that 225/30 = 7.5% damage increase. From the Hunter skill tree that I provided, 10SP was invested into Perfect Attack Bonus 1 which gives an additional 10% damage bonus on any Perfect Attacks you get. Assuming you have a high rate of Perfect Attacks, the damage multpliers you'll be getting from this Hunter sub skill tree will be 1.075 [all melee boosters] x 1.10 [Perfect Attack Bonus 1] = 1.1825 or 18.25% damage bonus.
Now that I've shown you how much damage you get from this Guard Stance focused Hunter sub skill tree, let's take a quick look at what you can get from a regular Fury Stance focused Hunter sub skill tree. Below, I will show the calculations for damage reduction provided by this tree as well as damage multipliers.
First off, damage reduction: 200 [damage received] x 0.94 [Flash Guard 1] x 0.75 [Hunter Physique] = 141 total damage received. This is gives us a total of 29.5% damage reduction (1 - [141/200]). With this figured out, let's move on to the damage multipliers.
A normal Fury Stance focused Hunter sub skill tree build will have the following damage multipliers. Keep in mind you may have inconsistencies with regards to having Mel-Atk Up in the current Hunter sub skill tree due to how you build it. For this case, I will exclude any Mel-Atk stat bonuses and focus solely on the multipliers.
1.20 [Fury Stance] x 1.05 [Fury Stance Up 1] x 1.05 [Fury Stance Up 2] x 1.10 [Perfect Fury Bonus] x 1.10 [Perfect Attack Bonus 1] x 1.10 [Perfect Attack Bonus 2] = 1.7609 or 76.09% damage bonus.
Now that I've shown you all the math, let's finally bring place them side by side in order to see and compare.
Guard Stance focused: 52.5% damage reduction, 18.25% damage bonus
Fury Stance focused: 29.5 damage reduction, 76.09% damage bonus
As you can see now, going Fury stance will provide us with about 30% damage reduction and a whopping 76% damage bonus to all of our Perfect Attacks while Guard Stance will provide us with 53% damage reduction and only a 18% damage bonus to our Perfect Attacks.
Lastly, since it is of utmost importance, I would like to briefly mention that Fighter is the only class that heavily relies on critical hit damage in order to dish out high damage output. This is due to their skill Critical Strike, which gives a multiplier of x1.15 to any critical hit. This means that as a Fighter main, you want to stack as much critical hit rate as you can in order to reach 100% for optimal performance. Unlike Guard Stance, Fury Stance will provide you with a +25% critical hit rate up from Critical Fury. This means that you'll have higher chances of landing critical hits which will allow you to use the Critical Strike multiplier of x1.15 to further increase your damage output.
I hope this was able to help enlighten you on how each Hunter stance compares to one another as a sub for a Fighter main. Like I always say, I'm all for player choice and playstyle as no one knows how you would have fun more than yourself, but above all else, I want to ensure that you are well-informed before you make any huge decisions such as this.