Well, there's two main ways you can go about it: the "normal sane playthrough" strategy, and the "trying weird stuff" strategy.
For normal play:
Your best friend is Bards combined with an upgraded ratfolk, or alternatively with an upgraded flying skull holding bones. Or later a summoner. Just that can hold an entire line on their own until the endgame, and even then all they need is the addition of a Rocker (in ANY lane!) except maybe in the final boss fight. The only downside to this strategy is that if you aren't careful it may result in triggering the final boss fight before you are ready!
Vikings and Stoners are good early game things, but will fall to mediocre as things progress. Skulls start out worse than either of those, but get much better with upgrades. Milkmen are situational; they can be great or nearly useless depending on the composition of the rest of your army. Bowies suck no matter what, ignore them. Stickies are fantastic units for how common they are, just because their upgrading is better than most.
The Bat Curse spell is fantastic. In early to mid game, it is good at killing whatever slips through your defenses, or just killing the most hard hitting stuff (Royalists and Guards) first before they can kill your units. In late game, it removes those pesky shields and bows.
When you reasonably can, calculate ahead of time what will kill what; this will best let you know when to use spells and on what.
Remember that Freeze is a thing!
Don't necessarily just use all your spells at the beginning of the fight; it is often better to keep some in reserve to use later.
Always go for Mausoleums, except maybe if you are about to fight the king.
Consider waiting until the third bone before upgrading anything to the third tier; this will ensure you get four bone stuff appearing to buy.
Use the Summon spell to block archers from taking a step that would enable them to use their bow. This can also potentially be used to make Snitches and APCs spawn only a single unit on death! Works best if you have a Rocker somewhere. Alternatively, use the Summon spell on a space occupied by your own units, to push them back; either to keep them safe from whatever enemy is in front... or to give kobolds and pigs more chances to do their thing!
A single succubus, if you upgrade her health a bit and support her with shielding and spells, is another thing that can easily solo an entire row outside of the final boss fight.
Sell all buildings just before the battle with the king, since you won't be needing them afterwards.
For the final battle, put your most powerful stuff in the middle row, Bat Curse the two princes (to remove their shields if nothing else), note that succubi are ineffective against the king and princes. The Blessing spell on your hardest hitting unit is good for surviving the king's blows long enough to kill him.
For trying weird stuff:
Don't even bother unless you can fairly consistently win with normal play.
Bat curse is even MORE useful here. Get at least one on the first round, reset if you don't get it. Maybe even don't bother with any actual units for the first couple rounds, Bat Curse books only. Eventually aim for a full set of 3 fully upgraded Bat Curses.
Reroll, a lot. Freeze, even more.
ALWAYS go for Mausoleums, they are what will eventually enable your weird stuff. (Especially if your weird stuff IS lots and lots of Mausoleums)
Focus on having the contents of each row be just a single tanky and easily upgraded unit. Vikings are good for this, Stickies are even better. Give them all Shields. Boost with Blacksmith and Farmer, aim for 9 attack to one-shot all non-boss units. Ignore most other units unless they are needed for whatever weird thing you are trying out. Once you have done this, you can stall for an extremely long time, enabling your Mausoleums to let you buy... whatever.