lul git gud, be more mlg!
Sorry, had to.
But with that out of the way, based on my experience with using Shingen on my water team, she's best used on a team that can achieve a Full Burst in 6 turns (preferably less for the first one), assuming you want to use Provisional Forest off cooldown and FB right away (or use it 2-3 turns after the battle has started and FB before its duration expires). You can accomplish this in a number of ways, depending on what hime you have access to.
For water, Asherah can give 20 BG to the entire party every 9 turns, helping you get the first FB quickly, while also providing a stacking 20%/30% Burst DMG increase to compliment/maximize the damage of your FB. Saraswati, on the other hand, can help bring any hime/souls falling behind up to speed with her combo buff, and can also help Shingen regain BG quickly after using Provisional Forest with her healing skill (assuming no one else needs it) even without Shingen's spear. Vohu Manah can insta-burst whenever and give everyone else 10BG, she can gain her BG back quickly with Guaranteed Triple, and she also has Water Res Down to help you reach defensive debuff caps.
Lakshmi (who needs her own spot to explain this, based on personal experience) can also be a pretty good option if you know what you're doing. She has a powerful passive ability (increased ATK, combo rates, and Abi/Burst DMG caps for each stack of Erosion up to 4), extremely powerful nuke skill (hits twice at level 75 with 3 erosion), and a powerful Burst (does additional echo DMG equal to half her burst DMG at 4 erosion), meaning that she can easily build up her burst gauge AND provide damage that water teams sometimes lack, especially against Fire (stacking Crit buff). However, she can easily become a glass cannon and die quickly if you're not paying attention (2.5% Max HP DoT and less healing received (potion healing seem unaffected) for each stack of Erosion up to 4). So you need to have good erosion control with her, and use her self regen/nullify/Erosion reset if you think she won't make it to the next turn.
For light, Michael AW is your go-to hime for speedy FBs, and it's suggested to almost always have a spot for her in your main lineup (not sure if this still applies before Awakening). Vishnu is also another option with her 5T/1CD stacking buffs, but be careful not to let her take damage or her momentum will come to a screeching halt (having a 0 show up won't cause her to lose her personal buffs).
Of course, if you have 15 Master Points and your team can handle the necessary debuffs without needing to use Shingen's EX slot, then get her Encourage Inspiration Mastery EX skill and use it. 20 BG on a 6-turn cooldown means that Shingen can easily speed up most teams by herself (and also get full BG after PF with spear), though extra combo buffs and BG generator certainly don't hurt if you happen to have them.
Edit: Don't worry about whether or not your accessories are good yet. As long as you've got 3 tiaras of any grade (preferably SSR) to equip on a hime with 5 acc slots (for the combo rate boost), you're good to go, and you can worry about getting better accessories later. As far as I've read, Ancient accessories aren't a HUGE upgrade over normal SSR accessories, so don't worry about not having them right now, and just focus on getting tiaras and other okay accessories (though obviously trash/fodder any extra/non-tiaras that have something useless, like Mode Gauge x3).