I like the idea of a big, bad boss bossing people around, but there is probably one thing that I feel need to be tweaked on Magnus before he can become a competitive threat, since he feels like he's suited to certain comps similar to how Juno is.

Primary Attack: Power Drive
Charges each turn the ability isn't used, up to a 3rd tier. Damage is spread across the 3 tiers similar to how Titus Ult works.
+ Modding for Slow is a boon since it allows him to get in range for his Knock Up ability
+ AoE capability
+ Mod for losing only one charge on hitting only the 2nd and 3rd tiers allows for pressure
- Charge time reduces Magnus' effectiveness each time he uses it.
- Damage is lackluster unless he is within 2 squares.

Decent Primary compared to other Frontlines since it gives him a semi-CC game he can play(considering he's heavily focused around CC).

Extinction Event
Places a 3x3 Targeter on enemies within the Targeter during Prep Phase, then blows up on the enemy on the next turn's Blast Phase.
+ Good for trapping and getting possible AoE pressure on Comps using Su-Ren or clumping freelancers
+ Pretty long range, and the mod for bypassing walls is amazing on certain maps
+ Can work as a read on dashes
+ Ignores Cover.
- Very long Cooldown of 4 turns.
- Damage is mediocre, but can be dashed out of in the next turn.

Very good 2 Ability to have, despite the long cooldown.

Horns Up
When activated, provides 10 Shields. If not used a subsequent turn, provides 20 Shields. If used on a subsequent turn, performs a Dash that does 26 damage and roots the first target hit.

+ Shield buffer helps protect from chip damage through cover
+ Rooting on dash allows for better kill security on targets with no dashes available
- 2-turn setup.
- Shielding is minimal unless both turns are used for the shields, making him especially weak to focus fire
- Cooldown is in fact 3, not 2 in-game, making the Cooldown mod a requirement to actually keep what the UI says.
- Dash can leave Magnus vulnerable when dashing against Cover(despite ignoring it on the first target hit).

This ability I believe should have its targeting fixed in a patch. Let alone, the problem that Magnus faces is that he has to choose between a dash that can possibly avoid damage, or a shield that can mitigate incoming damage. This trade-off is most times not worth it, but the set-up requirement makes it difficult for Magnus to compete with other Frontlines competitively. One thing I think would make sense is leave the Cooldown at 3 Turns and just allow him to dash straight-up and gain the root.
Fake Out Mod could be changed to Battering Ram to either increase damage by 4 or apply a Slow to all enemies hit by Horns Up.

Deal Breaker
Deal damage and knockback an enemy to a selected spot within range.
+ Very strong CC, especially since it's a native knockback that doesn't require a Mod.
+ Decent damage, can be increased to Firepower levels with Haymaker.
+ Allows for set-up and combos with a variety of freelancers
- 2-square range. Very difficult to use.

Very good ability to have in Magnus, solidifying him as a CC Frontline. Only real downside is the range, which is pretty crappy, but if he manages to get in range with teamwork he can deal a devastating blow.

[Ult] Boss Around
+ Strongest CC in the game, forcing enemies to Auto-Follow Magnus.
+ AoE ability at 100 Energy
+ Combos with Deal Breaker very well, and can be used for kill set-ups.
+ Weaken Mod very valuable due to the range
+ Good mod suite to suit his needs in match-ups
- CC used on Magnus reduces effectiveness or outright nullifies it
- Not as effective on large maps due to the size of the targeter

This is probably one of the best Ults in the game imo, mainly because it forces the enemy to Auto-Follow, The mods available allow him to either prevent himself from being countered by opposing CC or reduce enemy effectiveness.

Overall I think Magnus just needs a minor tweak on his Dash since it's really crappy considering the other Freelancers have dashes available(though Rampart has a shield which he can use) and on-the-fly shield mitigation doesn't make much sense unless it's 20 shields or more in a single turn if Magnus is the focus target of the enemy team.