The blue portals are mostly of simple mobs, especially at the beginning, with the mobs at the level you are at, except probably the very last blue portal(which is insane if you ask me.), though I can't be sure about the boss fights. At blue portal 20 or so and above are boss fights, except the 24th and last blue portal, which contains monsters that you probably have never seen, and you probably wont want to see again, just look at the stats at the bottom to find out what I mean.
Similar to the items Myuutan can collect, the blue portals is much more troublesome, as it reduces the number of characters allowed in a blue portal challenge by 1 each time your base gets closer to it. The maximum number possible is 6, and reduces to 5, 4 etc etc. Do bear in mind that you would take damage on the map while moving to the blue portal, so do weigh your pros and cons before deciding whether to move your base ahead.
During the course of your clearing of these blue maps, I would suggest you use at least 2 units with 4, if not 5 and above in movement. This is due to the existence of a non-attacking enemy unit that reduces your entire party movement by 1. I have had games where my group has 2 units of 2 movements, thus both reduced to 0 and locked up my entire party, thus a failure, and another where my only unit surviving after the battle has only 1 movement, and can't reach this enemy in time. Thus I would strongly suggest you kill that unit ASAP wherever possible. Another suggestion is to try to use more of physical attacking units, compared to magical ones, since there are maps where the map is filled with hannies, which has a 99999 magical defense.
There is a stage turn limit at the lower left corner. For the 20+ maps, this number is usually 10. When this hits the maximum value, you can consider it as a failed attempt at this blue portal. Even if you fail the map this attempt, it should be available again at the next slice of map you visit.
Some of the maps will give you new generals/characters to use. One of which is a lizard gunner, called "Prince of Monsters". If you pump him up to level 50 or so, a new character, his sister, "Princess of Monsters" will join your party. The Prince has 3 movement and 5 attack range, whereas the Princess has 4 movement and 3 attack range, both of which should prove rather useful.
For Boss fights, you probably would only want 1 or 2 melee units, since only 1 melee unit can hit the boss at any time. Portal Challenges(Can't be sure of the numbers, can't exactly recall in detail):
21th Blue Portal - Blue dolphin-like boss, relatively easy. Map has a hook to its face, which is where you need to hit. Suggesting a group of a good melee, and some ranged units.
22st Blue Portal - A tough monster, you need an entire team of hard hitting and good surviving characters. Reason being its attack hits ALL UNITS ON MAP. At map level 30, it has around 400 attack, with around 20k HP and around 100 defense 100 for both physical and magical defense. Suggest a far and high damaging group. I used two melees, one of which has 2 attack range boost, allowing him to act like a mid-far range attacker.
23rd Blue Portal - A big monster that is moderately hard, very high damage, 800+ at level 31, with quite a high HP, about 28k. Defense of about 200. Hits only single targets. Despite the high damage, your party will probably survive through the entire stage, compared to the previous blue portal. And you get a strange painting that I do not see any relevance to the game in any way. So much for the second last map.
24th Blue Portal(Last?) - At level 32, it consists of 10 flaming monsters, each of stats 9999 attack, 9999 physical defense, 9999 magical defense, Hp of 80,000 and above. Hit rate of 99, dodge rate of 15, movement of 3, attack range of 1. And you have 10 turns to kill them (roughly kill 1 per turn? if you can, that is.)