opt1: evocation cleric. lvl 20 pure, best race - dragonborn, okay options are human and aasimar.

Domains magic (more SP, evocation DC), Luck (less DC and SP but better survivability), or Air (two awesome SLAs. evocation DC).

Starting stats:
all into WIS, next into CON, rest to whatever you want (STR dump stat, DEX for better reflex, CHA for turning, INT for more skills) . Lvl up WIS.

Itemization: best you can get for spellcasting including: red fens set at7, slaver set at 8, GS items, Cannith Crafted stat items and SP items (stat items for lvl 4\5, 7, 13, 18, 23; sp items for every several levels. I prefer 4\8\12\16\20\25), ravenloft gear, especially belt.

Very important stats: spellfocus: conjuration, evocation, necromancy, enchantment (I prefer just spell focus mastery hats). Spell penetration, Spellpower and spell critical: fire, force, positive, light. Stats: Wisdom, Constitution. Wizardy item (+SP).

Feats at lvls:
1) maximaze
2) domain (air, luck or magic)
3) quicken
6) empower
9) spell focus: evocation
12) spell penetration
15 and 18 optionally depending on your pastlifes. If none, get greater spell focus and greater spell penetration. If have, get completionist and wizard pastlife, may also want heighten right there

Enchantments:
41 into DD, Emissary of light, of corse. Take all spell crits (universal till epics, on epics may switch to light), both +WIS, t4 and t5 SLAs, all spellpower (universal). You may either take single targer ray SLAs or efficient metamagic: quicken. Your choice.
If dragonborn, go deep in racial tree. Dong go cores past, take hp and some AC, all 3 tiers of arcanuum, dragon breath (fire), fire spellpower, draconic knowledge, memory of flight and +3 evocation DC.
If aasimar take all cores and arcanuum, maybe, healing amplification, nothing more. If Human, take core wisdom, action surge: wisdom, h amp.

Rest spend in Warpriest: gain hp, sacred touch, inflame, wall of steel and as many cores as you can.

RS tree not needed at all for solo until you play some quests with a lot of undead and handle to get okay CHA. so reset your warpriest tree and go for Mighty Turning for those. Will work on heroics, wont in epics.

Important spells: soundbirst, cometfall, command, greater command - are your CC spells affecting almost 100% of enemies (only exception are wraithes). Command spells also work against reapers. On epics take glacial wrath feat\spell.

Divine punishment, Nimbus of light, searing light: lame damage single target spells, but super cheap. Your only attack spells on lower levels, and you may have to use them until your 20 to carry some fat bosses.

Firestorm, Sunbirst, Sunbolt, Flame Strike, Cometfall again, Blade barrier - your heavy artillery spells.

Destruction, Implosion: Instakills. While not be able to use destruction on anything but enemy mages, implosion will work very well for sure.

Epics: glacial wrath, mass frog, energy burst twist from draconic incarnation and playing in Exalted angel for Divine wrath is essentional. Rest to your very own choice.

How to play:
Heroics till 12: slowly Get your first SLAs from DD and use them maximazed. Remember, maximaze and empower feats are only for SLAs (all from DD, domain or ED). Once you get soundbirst use it to make your enemies stunned and helpless, melee mobs, heal self. Wont be too terrible till 8-10 lvl. Once you get Holy smite, Holy smite SLA and Flame strike you'll find your self in very better position against numbers of mobs. Again, use sounbirst for helpless, spamm all aoes and enjoy power creeping
Once you get cometfall, firestorm and flame strike SLA you'll be fine on heroics.

On epics use all the same + energy burst (fire) and divine wrath, glacial wrath to make enemies helpless. If you are dragonborn dont forget to use dragon breath with empower and maximaze.

Evocation cleric was pretty fun to play for me, its very and very sorc or fvs-like. You can always respec it into healbot and be usefull at legendary content aswell. Yet leveling ability of this guy on epics was frustrating. Outside epic hard I felt really uncomfy, cause cleric spells from book do no damage on r1 and higher at level on epics. So it was about to wait energy burst\divine wrath CD and implosion.

P.S. If you go air domain you dont need to get lighning spellpower, it will use light. Yet spell critical still lightning, so if you able to slot it, do it.

Opt 3: necro cleric.
Since you want to solo things it'll be very similar to evocator one but with some changes:
Domain: death
Feats:
1) maximaze
3) quicken
6) spell penetration
9) spell focus necro
12) greater spell focus necro
15) greater spell penetration
18) heighten

In DD go darkness, take universal spellpower, both necro DC. SLAs mostly useless. Clerics have only 3 necro instant kill spells: destruction, destruction SLA and slay living. Yet cooldowns are decent and DCs are impressive, so you can easily solo most not very mob swarmy quests even on high heroic skulls. For swarms, bosses, deathblocked champs still use your regular direct damage spells over CC/helpless

Build agnostic, you can land turns with the very obvious turn gear in heroic. The necklace from Ravenloft is enough to get you landing without otherwise building for it. But Death domain is gravy. Solo the undead heavy content if the build is a late bloomer or whatever. RS Mighty Turning until it blooms then reset.