General Guan is based off of Guan Yu, known most commonly from his role in the Three Kingdoms.
In Japanese, he is known as Kan U, which is why his katakana name is "General Kan."
As you might expect, there are many adaptions of the Three Kingdom and their characters, so there are many adaptions of General Guan.
General Guan has a powerful combination of abilities.
First, he has Attack First, which is always strong. On top of that, his ST can grow infinitely. The only other creature that has this combination of abilities is Mothman, but unlike Mothman, Guan has 40MHP, allowing him to survive Evil Blast, and no territory restrictions, which is very important as Guan's main use is the invasion of territories.
His ST growth is the Number of Creatures on the Map x 5. 5 itself is not a big number, with Garuda's 10 or Red Cap's 20. However, it is not limited to your monsters and not limited to an element, so you can bring creatures from other elements into your book. However, still, it is true that with only x5, General Guan will not easily reach the three digit area.
His greatest weakness is with no doubt his item restriction, Tools. Why in the world would a humanoid creature have a Tool restriction (not including the all-brawn, no-brain Gladiator)?
The only real reason is to prevent General Guan from being too strong. Now don't you wish Garuda could have a bit of that restriction?
Anyways, the consequence of not being able to use Tools is quite heavy. That means no Gremlin Amulet or Black Orb. This makes General Guan very weak against Negation/Reflection. Thankfully, he can use Scrolls. However, it is important for you to destroy your opponents' negation items before you invade.
From experience, I feel that General Guan works well in a book just for him. He actually doesn't do that great in a simple Fire book, although there are quite a number of creatures with 50+MHP that do not have summoning costs. I guess the main reason is that those creatures aren't particularly great under Ares' influence. General Guan himself is strong under Ares because of the reason any Attack First creature is strong under his influence, but the other 50MHP creatures do not have attack first. Yes, Chimera exists, but he has a summoning cost.
Books that use lots of high MHP creatures like Support books, especially Son Ascetic books, also could benefit from General Guan's Attack First high invasion power. Though he still won't reach that high of a ST.
My Guan book personally uses Ares, Quetz, Giant Eel, Master Monk, Hurricane, Sleipnier, and Ba=al@Conjurer.
So anyways, cards that work particularly well with General Guan:
One of the most feared Boost creatures in the game. By simply positioning Ares on the map, Guan will gain +25ST during invasion.
Guan himself can protect against invaders thanks to his Attack First.
50MHP creatures that work with Ares include creatures with Negation, such as Giant Eel, and Attack First creatures, such as Master Monk.
Ares himself can act as your high level territory, as battles involving Ares will not include the ST boost.
Positioning Quetz on the map will increase Guan's ST by 15. By positioning both Quetz and Ares, Guan will have +40ST.
Considering Ares needs two Fire territories and Quetz requires 1 Wind, you can also assume that you have at least two other creatures positioned on the map. This means that by the time you position these two boost creatures, Guan will have at least +50ST during invasion. Very powerful, but also very heavy, as there is so much territory requirements to get these two on the map.
To move your creatures and invade territories.
Sleipnier has only 40MHP, so it won't boost Guan's ST unless you use Growth Body on it. Which isn't a bad idea, if your book has space.
The ever-so-handy invasion book weapon. A big costly, but you can use it over and over again, and both +20ST and +10 goes a long way.
Every ST+ effect pretty much doubles when you use this weapon. Ares becomes +40, Quetz +20. Two MHP creatures + Ares + Quetz + Tonfa on Guan reaches a total of 160, which is enough to pierce through Colossus + Lv5 + Support creature and Diamond Armor + Lv5 + MHP50 creature.
As Guan's ability is not =, but +, by using Turnover, Guans on the map already will gain a +20 boost. Other MHP50 creatures like Giant Eel and Master Monk also receives a boost. Beware not to use it twice, or you will cancel the effect of the first.
Creatures that do not help Guan must perish!
Annoying MHP30 creatures and below become paralyzed, allowing you to easily take them down. Their Land Protection ability is also neutralized, so you can reach them through Sleipnier and Chariot too!
List of Creatures with MHP 50 or higher:
Underline = No Summoning Cost
Italics = Attack First or Negation ability
Bold = Restriction of Armor (In other words, cannot use the protection items Guan can use)
Will o' Wisp
Ba-al (from Conjurer)
Pillar of Flame
Wall of Stone