YMMV / Dragon Island Blue

• Annoying Video Game Helper: The Spirits, once called, will act on their own accord. In most cases, the worst they can be is useless, but a few of them have abilities that can cause problems for you directly - most obviously, 'Resistance', the first spirit you get. His 'Nap' spell targets both friends and foes for Sleep-status, so if you're a bit unlucky, he can wind up putting all your monsters (and himself) to sleep, while leaving the enemies free to pound on you. Bringing him out in a boss-fight (bosses being generally immune to status-effects) is downright suicidal. However, a couple of the later Spirits - Karma and Divine - can also cause trouble despite being generally quite powerful. Their 'Neutralize' spell removes all effects on the enemy team, which can be great if they've buffed themselves up - but usually just ruins whatever debuffs and ailments you've been hard at work laying on them.
• Demonic Spiders: Unhappy Birds. They die easily enough, but that's because their main attack is to suicide bomb your team!
• Goddamned Bats: The Megaslugs. Near the end of the game, you run into MOBS of them... 3-9 to an encounter. They're not likely to kill you, but they sure as heck will annoy you. Two main reasons: Firstly, they're 'Tank' creatures, which means that they'll randomly redirect attacks aimed at team-mates towards themselves. When facing 3 tanks at the same time, that means you've got little to no control over where your attacks go, and thus can't focus on whittling down one at a time. (And, of course, they've got tons of defense and HP. Secondly, they've got the dreaded 'Nap' power. This hits the whole battlefield - your monsters AND theirs - with Sleep-status. Since the primary way to snap out of sleep is to get hit by the enemy... this leads to long periods of absolutely nothing happening while both you and the enemies snooze away. Add to that the fact that they primarily appear on the road directly outside the last city - which means you'll mainly be running into them just as you're leaving for some epic battle or climactic showdown. Oh, and if they finally decide to actually attack you, they'll probably one-shot a few of your frontline troopers... due to their special 'Survivor' attack, which grows more powerful the longer they stay in battle. Including time spent sleeping. Thoroughly annoying.
• That One Boss: A couple, mostly in the big post-game quest, or the highest level of the Arena Tournaments. Interestingly, most of them effectively use a Zerg Rush tactics, simply unleashing an enormous horde of identical monsters that happen to work particularly well in crowds. One will assault you with 38 Wolf Gods (mentioned above under Magikarp Power as the cheapest unit in the game, due to its high speed and Stun-inflicting hit-all attack), while another uses a similar number of Morfeus-monsters, each of whom will either use a hit-all Sleep-spell, or unleash 'Dream Hunter' which deals enormous damage to your entire team if even a single monster is asleep. In both cases, you have the choice between A: Building up a varied team that's powerful enough to overcome them or B: Doing the same thing THEY'RE doing, but BETTER.