Momentum is likely the most interesting ability on the Crusader. By building up a damage bonus, it's often possible to take on a higher level monster than would be possible without it. Its bonus stacks additively with Attack Powerups and with the Human's attack up on glyph conversion. The bonus is lost upon attacking monster or using BURNDAYRAZ on monster, without killing it, and goes up by +10% every time a monster is slain. As the bonus does not affect BURNDAYRAZ, it is generally best to start a fight with a physical attack if any bonus is active.
Martyr is a good ability to have against the Boss of the dungeon, or the second boss in the case of challenge dungeons, since if the damage wins the dungeon for you, you still win the dungeon. It is useless at any time besides that though, as attempting tricks such as having Death Protection from CYDSTEPP will not allow the attack from Martyr, although you may be able to use more potions earlier in the dungeon, planning for a slightly easier boss. Be aware that it deals damage based on your base damage though, not your damage as modified by Attack Powerups and Human glyph conversions. Also bear in mind that higher-level monsters will strike first, which means that you most likely won't get to land your normal attack before your Martyr damage decides the outcome.
Race and Deity Choices
Human is a good choice, as it is a little easier for them to chain kills together. Other choices of race such as Dwarf can be effective, but do not have as much synergy with the Crusader's abilities. As with the Fighter, an Elf may be considered for the Crypt, with the Crusader having the advantage that he can add a little damage against Wraiths with his physical attack, without being mana burned.
The Crusader gets the most benefit from Gods capable of increasing his base damage, such as Taurog, Jehora Jeheyu, or Dracul, although all have their drawbacks which should be considered based on the glyphs available in the dungeon. Anyone can benefit from The Pactmaker's abilities, and the Crusader is no exception, as getting bonus experience can lead to having more enemies left over to use with Momentum. The Glowing Guardian is always good for the healing, but, due to wanting to use Momentum, it may be difficult to stay with him. Tikki Tooki can be easier to generate piety for while using Momentum, although only his higher boons are particularly useful.
Factory is a difficult dungeon for the crusader, since both Animated Armours and Meat Men anti-synergize with the Crusader's Momentum ability. If you can find BRNDAYRAZ (and possibly BLUDTUPOWA) early, start taking down the Death Protections of any Animated Armour encountered in the dungeon, only killing the highest-level one for Experience Catapult. At level 6-7, you will hopefully have a good amount of lower and mid-level Animated Armours and Golems to sweep for the Momentum damage bonus. Keep in mind that the bonus only increases for the first ten monsters killed in a row - the eleventh consecutive kill doesn't increase the damage bonus.
If you can find Jehora Jeheyu early, it's worth worshipping to get the Polymorph boon - since two of the three classes of monsters in the Factory inhibit one-shot kills, Polymorph is likely to present a better selection of monsters to feed into the Crusader's Momentum machine.