WeldoRubin: Thank you! Following your advice I beat the last dark elf mission without having to cheat! I really appreciate it. Do you have any advice for the Elf campaign? Like what are the best level upgrades to choose?
Congrats! I take it you used the "shortcut"?
For the Elf campaign, Findan isn't able to get the Sylvan ultimate skill, so that's out. Kind of a pity because Deadman's Luck is actually worthwhile; it competes with Academy's "All Spells Known at Expert" for second best racial ultimate. Instant Gating from the Inferno takes top slot with no contest as far as I'm concerned.
That said, Findan can get truly hideously ridiculous in an awesome way. My build included Enlightenment and Logistics of course, and I went the way of Triple Ballista and Destructive Magic. So you're looking at Attack, Enlightenment, Logistics, War Machines, and Destructive Magic.
Happily, Destruction is one of the two primary magics for Sylvan, and the Sylvan campaign features plenty of Elf towns, so you're pretty well guaranteed to get Meteor Shower and Implosion at some point as long as you put in the effort.
In the later stages of the campaign, Findan starts off major battles using Imbue Arrow with either Implosion or Meteor Shower, depending on the circumstances. Then the ballista proceeds to wipe out stack after stack with three shots on each action, every shot dropping an Implosion or Meteor Shower on the unfortunate victim. If you attach Meteor Shower, make sure you target the highest health unit in the aoe, because if the target stack dies after the second shot, the third shot won't fire and thus the third meteor shower won't rain down on all the nearby targets. Also make sure to keep your guys out of those meteor showers! And watch your mana, I guess, but Sylvan level ups tend to include a lot of Knowledge.
I've attached a screenshot of my Findan from the opening of the 5th mission. He had stats of 28/27/15/23. If you had so much trouble with the last Dark Elf mission, then this one will very likely be challenging as well (though you do have towns so you can reinforce), so make sure you have Findan gather as much power as he can in the prior missions (visiting all stat bonus sites, collecting all possible spells, etc). Also put in effort for Talandar and Dirael if/when you have them on earlier missions; they'll be in Sylvan-5 as well.
The important subskills are War Machine's Ballista, Imbue Ballista, and Triple Shot, and the Elf racial skills Imbue Arrow and Rain of Arrows. Other very strong subskills are Attack's Archery and Battle Frenzy. Logistic's Silent Stalker was handy for the huge view range, and Pathfinding is always handy for extra movement speed.
Do your utmost to keep your Master Hunters and Druids alive. Sprites are also very useful with AOE No Retaliation and a weak Wasp Swarm to use while the enemy forces cross the map. Ancient Treants will probably be your front line with Unicorns serving a back-up tank role. War Dancers should slip in where ever they can do damage without being targeted, while the Emerald Dragons are an excellent damage unit that also happens to be immune to Meteor Shower (I don't think Green Dragons are immune though! Don't mix them up!).
Last note: For Sylvan-1, you should see a Necromancer hero show up every couple of weeks(?) and assault you. Biara should appear at the start of month 2 or 3 (I think) and charge you. If that isn't happening, then your game's scripts might be broken and you're going to need to get the cheats working so you can cheat the mission as completed. I wasn't able to complete the mission normally because Biara never appeared, so I wasn't able to complete the hidden required mission objectives. Iirc, my solution was to basically clear my side of the map, then cheat the mission complete; I watched part of a youtube run to see what I missed story-wise. From mission 2 onward, everything worked.