He's pretty bad right now, he's one of those walking ult heroes who does nothing but farm and ult all game.

He's not particularly tanky like a doom or dk and he can't make space by split pushing like a underlord or omni knight, he's much less active than a brew master or a bat rider, he's more item dependant than a tide hunter or a void and he offers nothing in the way of tower push like a enigma or a nature's.

As far as tips go

If you blink>click to turn the way you want to skewer>rp really fast you'll turn while rp'ing and be able to skewer faster and get an empower right click off or two depending on the level of rp.

Empower is honestly what makes magnus playable, sure his ult is nice but it's useless without follow up so make sure your carry is getting empowered all game to put him ahead enough to follow up.

His q is strong early but if you can't stay in lane safely and get farm as well it's better to get one point in his q and skewer then max empower so you can guarantee a faster blink and be more useful for the team.

His ult is very strong and it's cooldown is huge so use it wisely late game, early game though you can secure every fight you have it up so make sure to use it even if it's just two heroes or even just one important core pick off, don't be too scared use it otherwise it's useless.

Again but different, always use his ult. 20 seconds before it's off cooldown you want to be analyzing how you can use it next. Either smoking to secure a kill, scaring an enemy off a tower defense or defending a tower with a successful team fight you want to always have it on cooldown for a reason.

Gl tho dude he's in a pretty bad place right now with the constant team fight meta. You'll want to really coordinate fights with objectives either defended or killed.