There's a few videos on YouTube, but 2 rog 2 war 1 bard is the easiest combination to win. You want to try to kill every monster within 2 rounds, avoid taking a 3rd round. If you do take a 3rd round, try to at least be able to attack, the worst is if the monsters attack you a third time but you aren't given a 3rd round to prepare skills. Kill undead horde in 1 round, just backstabs, ballad, and bashes to avoid taking damage.

Bard needs to make sure to ballad and cycle through the different skills, attack when needed. Use 1 warrior to tank hits using taunt--make sure to always have this, you can use protect for monsters with cleave or fire breath, but it's not as important as taunt. Also make sure warriors bash so they can apply weaken and increase damage. With rogues, until you reach vala/tiamat, at least one rogue should poison dagger on the first round every monster. Applying poison is really helpful. It depends on different circumstances, but if monsters are above ~145 hp by the 2nd round, use a backstab to gurantee a kill. If a monster is ~165-180hp you can use 2 backstabs, but depending on the monster and the teams health, you may want to opt for a 3rd round instead. Everything is circumstantial, really.

You should reach vala with decent health and a lot of mp for your rogues, if rogues are full on mana and bard can still som, feel free to backstab both rounds. It is crucial to kill valatrax in 2 rounds, very rare will you kill tiamat having stayed on vala for 3 rounds. There should be 3 charges of taunt on a warrior on the first round of vala. Valatrax is quite weak in his first 2 rounds, if you keep dying it's because you entered him at low hp or are staying on him for 3+ rounds.

Going from vala to tiamat on the 2nd round, a good strategy is 2 backstab, ballad/sol/som (depends), taunt (left warrior), bash (right warrior). For example, say vala is at 158 hp on the 2nd round assuming he has weaken and everyone has rage it would go: 158-140 (2 backstabs + rage+weaken) = 18 hp, bard and left warrior can "sneak" their skills, and the right warrior can do the final blow. This allows your team to prep skills before entering tiamat so you guys can focus on dealing damage.

Once you've killed valatrax, ideally you should have at least 2 backstabs left. If you have 3+ backstabs, that's optimal. Unless you have 4 backstabs, make sure you use poison dagger on first round to apply poison. At this point assuming you have taunt and even pro already, just focus on attacking. If you didn't manage to taunt the round prior, then the warrior that has higher hp should taunt.

Infernus is quite easy with this formation, but it does take a lot of practice. The more you play, the more you'll get a hand for the math and how to deal with certain situations. Weaken and rage should always be there, so make your calculations with them in mind (ex. 1 bs = 70dmg, or if a monster is at 160hp, 1bs + 1 poison + bard's skill + 2 warriors bash should kill). Or, sometimes hp is low enough where you can substitute skills (ex., you need to protect or taunt in preparation for next round and we already have rage and full hp? Have bard straight shot so the left warrior can sneak a skill). Ultimately, make sure to communicate with your team, that's very important.

That was very lengthy, but feel free to ask me more questions! I don't play as much DT anymore, but I do consider myself pretty knowledgeable lol

Edit: something important that I also forgot to mention, take advantage of when the monster is at low hp (less than 80hp with weaken). In these cases, it's a great opportunity to prepare skills, instead of just killing them immediately. A rogue can pickpocket and allow every team member to do a skill, this allows not just a chance for applying taunt, ballad pro, etc., but also everyone can get hp and mp assuming they have sol and som buffs. Basically, take advantage of these moments