In general, the assassin is most effective if it is leveled up quickly, as it can clean up remaining monsters quite easily. You should be able to consistently reach level 10 by the time the map is cleared.
If you have fireball, find a high level enemy with very little health such as a Gorgon, cast poison on it, and then explore to regenerate mana for fireballs until it is dead. Otherwise, find a high level monster that is unable to kill you in one hit, and beat it in a similar way but with normal attacks. CYDTEPP can be used with the mana that you are regenerating to score additional attacks. If you are using magic exclusively while doing this, BLUDTUPOWA can be used to allow for faster mana regeneration rate. If all of your paths of exploration are blocked and you have 5 mana, kill as many of the units that are blocking paths as possible, and then recast poison on the unit that you are trying to kill
At the beginning of the game, attempt to kill a level 2 enemy, since this will reduce the number of enemies that you have to kill to reach the second level, and thus leave more targets for later. After level 1 try to attack high level enemies even if doing so will require a potion or a lot of exploring, since doing so will cause you to be a higher level than most enemies and leave you with more targets to kill later on.
Keep track of how much experience you can gain from your collection of lower level enemies that you are leaving untouched, and when in a fight with a strong enemy and especially with the boss, see if you are able to level up in the middle of a fight by killing these low level enemies and, if possible, how many times you can level up. Try to save some of these monsters for the boss fight.
When picking lower level targets to destroy for experience, try to choose those that are blocking the way to unexplored area to maximize your exploring potential later.
Begin to attack the boss at about level 7 and level up whenever you need to heal in the middle of the fight from the enemies that you have kept remaining until you are level 9 or 10.
Race and Deity Choices
Playing as an Orc gives you additional experience on demand, and allows you to level up faster. Try to save glyphs that you intend to convert to experience until the conversion will aid in leveling up in the middle of a fight. Playing as a Dwarf allows you to sustain more damage, which means that stronger enemies will not kill you in a single hit as easily. Playing as a Human will give you more attack with each hit, so you can finish the long poison battles faster or are able to kill units in a single hit with first strike more easily.
The Pactmaker gives additional experience for every kill, which is very useful when trying to level up in the middle of a battle from existing lower level units and also allows you to level up faster. Tikki Tooki will be pleased with assassins quite often for the displays of skill of killing lower level enemies. Dodge chance is useful in making long fights work to your advantage. That said, most of the other benefits offered are easily replicated by the assassin. Jehora Jeheyu will grant you piety whenever you deal or receive damage, which happens a lot if you are partaking in long poison battles. Note that you will be unable to use CYDSTEPP and GETINDARE glyphs unless you're prepared to deal with Mana Burn. Taurog is good if you follow him early, but you will be unable to rely on poison. Instead you will likely be focusing on power kills with first strike or using levelups to heal you. Amusingly enough, Mystera Annur doesn't disable your Swift Hand, meaning you can still use physical attacks to kill low-level enemies under her. This does come at a piety cost, but it makes her a surprisingly viable choice for assassins, especially as they start with APHEELSIK.
This is not much of a challenge. Start off by killing Gorgons 1-3 levels above you to quickly reach level 7. Make sure, that there are still enough tiles for regeneration and begin attacking Medusa. Be careful, since you need to have 100% health (or protection) to not get petrified. There should be enough monsters around to level up midfight for a quick heal. If needed use a few potions. Afterwards Jormungandr can be outlasted with your remaining potions and unused tiles (HALPMEH can be a big help here, too).
It doesn't matter so much here, which race or deity you take here. Orc, Human or Gnome all have their benefits, but none of them essential. For as gods I would think about taking The Earthmother or Glowing Guardian, due to their poison immunity, making Medusa and Jormungandr easier to kill.
Undead units are immune to poison, so you will have to manage without it. Use mid-fight levelups whenever possible to take on the highest level enemies. Vampires are probably going to be your main targets since they have relatively low maximum health. Since you're carrying a deadweight glyph right off the bat and mana burn will be prevalent, Taurog should be even more valuable. A couple fireballs followed by a first strike attack or a CYDSTEPP attack will be ideal for taking on strong enemies.
The bosses in this stage can get annoying due to their high strength. Level up as fast as possible with high level monsters, using poison and tile regeneration. Be sure to keep useless Glyphs for Exp conversion (assumed you play as an Orc), they will be needed later.
Start attacking the Matron of Flame on level 7 and use the remaining tiles and mid-fight level ups for regeneration(if you have HALPMEH or CYDSTEPP it will make things a LOT easier). Afterward you should be Level 8-9 (or maybe even 10). If you didn't use any potions so far, you should have no problem outlasting Aequitas.
Use fireball (and BLUDTUPOWA if you have it) to kill the highest level Animated Armour that you can find to get you ideally from level 1 to 6 in a single kill. Also, waste any other Animated Armour's protection from death without actually killing them whenever possible in order to leave them as potential targets for experience later. Be careful to not over-explore because you will likely want to poison Super Meat Man and heal from the black while fighting him. The Iron Man has a 75% chance to block poison when you cast it on him, so you can use it repeatedly on him until it works or try to beat him without it. When fighting him while he is poisoned, do not expect to be able to recast poison on him after attacking somebody else as it will probably fail. HALPMEH will be the ideal use of magic when fighting him.
Be aware that you can't kill lower level Animated Armours in one hit, because of their protection from death.